U.S. patent number 7,750,175 [Application Number 10/924,730] was granted by the patent office on 2010-07-06 for organic light-emitting diodes and related hole transport compounds.
This patent grant is currently assigned to Northwestern University. Invention is credited to Qinglan Huang, Tobin J. Marks.
United States Patent |
7,750,175 |
Marks , et al. |
July 6, 2010 |
Organic light-emitting diodes and related hole transport
compounds
Abstract
New organic light-emitting diodes and related hole transport
compounds and methods for fabrication, using siloxane self-assembly
techniques.
Inventors: |
Marks; Tobin J. (Evanston,
IL), Huang; Qinglan (Libertyville, IL) |
Assignee: |
Northwestern University
(Evanston, IL)
|
Family
ID: |
35968303 |
Appl.
No.: |
10/924,730 |
Filed: |
August 24, 2004 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20050234256 A1 |
Oct 20, 2005 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
10099131 |
Mar 15, 2002 |
6939625 |
|
|
|
09187891 |
Nov 6, 1998 |
6399221 |
|
|
|
08673600 |
Jun 25, 1996 |
5834100 |
|
|
|
Current U.S.
Class: |
556/424; 556/413;
556/489; 564/434; 556/465 |
Current CPC
Class: |
C07F
7/12 (20130101); H01L 51/0069 (20130101); B82Y
10/00 (20130101); H01L 51/0012 (20130101); H01L
51/007 (20130101); B82Y 30/00 (20130101); H01L
51/0094 (20130101); H01L 51/0595 (20130101); H01L
51/5012 (20130101); H01L 51/0059 (20130101); H01L
2251/308 (20130101); H01L 51/0078 (20130101); H01L
51/0036 (20130101); H01L 51/0043 (20130101); H01L
51/5048 (20130101); H01L 51/0035 (20130101); H01L
51/0081 (20130101); H01L 51/0077 (20130101); H01L
51/0034 (20130101); H01L 51/5088 (20130101); H01L
51/0039 (20130101) |
Current International
Class: |
C07F
7/10 (20060101); H01L 51/54 (20060101) |
Field of
Search: |
;556/413,424 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Grant & Hackh's Chemical Dictionary, 5th ed., McGraw-Hill, Inc.
(1987), p. 24. cited by examiner.
|
Primary Examiner: Yamnitzky; Marie R.
Attorney, Agent or Firm: Reinhart Boerner Van Deuren
s.c.
Government Interests
The United States Government has certain rights to this invention
pursuant to Grant Nos. N0014-95-1-1319 and DMR-00769097 from the
Office of Naval Research and National Science Foundation,
respectively, to Northwestern University.
Parent Case Text
This application is a continuation-in-part of and claims priority
benefit from application Ser. No. 10/099,131 filed on Mar. 15,
2002, and issued as U.S. Pat. No. 6,939,625, which is a
continuation-in-part of application Ser. No. 09/187,891, filed on
Nov. 6, 1998 and issued as U.S. Pat. No. 6,399,221, which is a
continuation-in-part of application Ser. No. 08/673,600, filed on
Jun. 25, 1996, and issued as U.S. Pat. No. 5,834,100, each of which
are incorporated herein by reference in their entirety.
Claims
We claim:
1. A hole transport compound having a formula ##STR00003## wherein
Ar is arylene; n is an integer from 1-4 and R.sub.5-R.sub.8 are
independently selected from H and a moiety comprising a
hydrolyzable silane, wherein the hydrolyzable silane comprises
three hydrolyzable groups, and wherein at least two of
R.sub.5-R.sub.8 are a moiety comprising the hydrolyzable
silane.
2. The hole transport compound of claim 1 wherein Ar is phenylene
and n is 2.
3. The hole transport compound according to claim 1 wherein each
moiety comprising the hydrolyzable silane is a hydrolyzable silane
alkyl group.
4. The hole transport compound according to claim 3 wherein the
silyl portion of each silane alkyl group is selected from a
trihalogenated silane and a trialkoxylated silane.
5. The hole transport compound according to claim 4 wherein R.sub.5
and R.sub.6 are hydrogen and R.sub.7 and R.sub.8 are selected from
trihalogenated silane alkyl and trialkoxylated silane alkyl.
6. A hole transport compound having a formula ##STR00004## wherein
Ar is arylene; n is an integer from 1-4 and R.sub.5-R.sub.8 are
independently selected from H and a moiety comprising a
hydrolyzable silane, wherein the hydrolyzable silane comprises
three hydrolyzable groups, and wherein at least one of
R.sub.5-R.sub.8 is the moiety comprising the hydrolyzable silane,
wherein the hydrolyzable silane is a hydrolyzable silane alkyl
group.
7. The hole transport compound of claim 6 wherein Ar is phenylene
and n is 2.
8. The hole transport compound according to claim 6 wherein the
silyl portion of each silane alkyl group is selected from a
trihalogenated silane and a trialkoxylated silane.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to organic electroluminescent
devices with organic films between anodic and cathodic electrodes,
and more particularly to such devices and methods for their
assembly using the condensation of various silicon moieties.
Organic electroluminescent devices have been known, in various
degrees of sophistication, since the early 1970's. Throughout their
development and consistent with their function and mode of
operation, they can be described generally by way of their physical
construction. Such devices are characterized generally by two
electrodes which are separated by a series of layered organic films
that emit light when an electric potential is applied across the
two electrodes. A typical device can consist, in sequence, of an
anode, an organic hole injection layer, an organic hole transport
layer, an organic electron transport layer, and a cathode. Holes
are generated at a transparent electrode, such as one constructed
of indium-tin-oxide, and transported through a hole-injecting or
hole-transporting layer to an interface with an
electron-transporting or electron-injecting layer which transports
electrons from a metal electrode. An emissive layer can also be
incorporated at the interface between the hole-transporting layer
and the electron-transporting layer to improve emission efficiency
and to modify the color of the emitted light.
Significant progress has been made in the design and construction
of polymer- and molecule-based electroluminescent devices, for
light-emitting diodes, displays and the like. Other structures have
been explored and include the designated "DH" structure which does
not include the hole injection layer, the "SH-A" structure which
does not include the hole injection layer or the electron transport
layer, and the "SH-B" structure which does not include the hole
injection layer or the hole transport layer. See, U.S. Pat. No.
5,457,357 and in particular col. 1 thereof, which is incorporated
herein by reference in its entirety.
The search for an efficient, effective electroluminescent device
and/or method for its production has been an ongoing concern.
Several approaches have been used with certain success. However,
the prior art has associated with it a number of significant
problems and deficiencies. Most are related to the devices and the
methods by which they are constructed, and result from the
polymeric and/or molecular components and assembly techniques used
therewith.
The fabrication of polymer-based electroluminescent devices employs
spin coating techniques to apply the layers used for the device.
This approach is limited by the inherently poor control of the
layer thickness in polymer spin coating, diffusion between the
layers, pinholes in the layers, and inability to produce thin
layers which leads to poor light collection efficiency and the
necessity of high D.C. driving voltages. The types of useful
polymers, typically poly(phenylenevinylenes), are greatly limited
and most are environmentally unstable over prolonged use
periods.
The molecule-based approach uses vapor deposition techniques to put
down thin films of volatile molecules. It offers the potential of a
wide choice of possible building blocks, for tailoring emissive and
other characteristics, and reasonably precise layer thickness
control. Impressive advances have recently been achieved in
molecular building blocks--especially in electron transporters and
emitters, layer structure design (three versus two layers), and
light collection/transmission structures (microcavities).
Nevertheless, further advances must be made before these devices
are optimum. Component layers which are thinner than achievable by
organic vapor deposition techniques would allow lower DC driving
voltages and better light transmission collection characteristics.
Many of the desirable component molecules are nonvolatile or poorly
volatile, with the latter requiring expensive, high vacuum or MBE
growth equipment. Such line-of-site growth techniques also have
limitation in terms of conformal coverage. Furthermore, many of the
desirable molecular components do not form smooth, pinhole-free,
transparent films under these conditions nor do they form
epitaxial/quasiepitaxial multilayers having abrupt interfaces.
Finally, the mechanical stability of molecule-based films can be
problematic, especially for large-area applications or on flexible
backings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B show structural formulae for porphyrinic compounds
which are illustrative examples of compounds of the type which can
be used as hole injection components/agents in the preparation of
the molecular conductive or hole injection layers and
electroluminescent media of this invention. In FIG. 1, M is Cu, Zn,
SiCl.sub.2, or 2H; Q is N or C(X), where X is a substituted or
unsubstituted alkyl or aryl group; and R is H, trichlorosilyl,
trialkoxysilyl, or a moiety having 1 to 6 carbon atoms which can
include trichlorosilyl or trialkoxysilyl groups, substituted on the
C.sub.1-C.sub.4, C.sub.8-C.sub.11, C.sub.15-C.sub.18 and/or
C.sub.22-C.sub.25 positions. In FIG. B, M is Cu, Zn, SiCl.sub.2, or
2H; Q is N or C(X), where X is a substituted or unsubstituted alkyl
or aryl group; and T.sub.1/T.sub.2 is H, trichlorosilyl,
trialkoxysilyl, or a moiety having 1 to 6 carbon atoms which can
include trichlorosilyl or trialkoxysilyl groups.
FIGS. 2A-2C show structural formulae for arylamine compounds which
are illustrative examples of compounds of the type which can be
used as hole transport compounds/agents in the preparation of the
molecular conductive or hole transport layers and
electroluminescent media of this invention. In FIG. 2A, R.sub.2,
R.sub.3 and/or R.sub.4 can be H, trihalosilyl, trialkoxysilyl,
dihalosilyl, dialkoxysilyl, or a moiety having 1 to about 6 carbon
atoms which can include dialkyldichlorosilyl, dialkyldialkoxysilyl,
trichlorosilyl or trialkoxysilyl groups substituted anywhere on the
aryl positions. In FIG. 2B, Q.sub.1 and Q.sub.2 can be substituted
or unsubstituted tertiary aryl amines, such as those described with
FIG. 2A; and G is a linking group to include but not limited to an
alkyl, aryl, cylcohexyl or heteroatom group. In FIG. 2C, Ar is an
arylene group; n is the number of arylene groups from 1- about 4;
and R.sub.5, R.sub.6, R.sub.7, and/or R.sub.8 can be H,
trihalosilyl, trialkoxysilyl, dihalosilyl, dialkoxysilyl or a
moiety having 1 to about 6 carbon atoms which can include
dialkyldichlorosilyl, dialkyldialkoxysilyl, trichlorosilyl or
trialkoxysilyl groups substituted anywhere on the aryl
positions.
FIGS. 2D-2E show structural formulae for numerous arylamine
compounds, in accordance with this invention, of the type which can
also be represented by the formulae of FIGS. 2A and 2C, such
compounds which can be used as or in conjunction with hole
transport layers. With respect to FIGS. 2D-E, each of Ar.sub.1-4 is
independently an arylene (i.e. as understood in the art or as shown
and used herein, e.g., substituted and unsubstituted phenyl, aryl,
naphthyl, anthryl, phenanthryl, or other polycyclic condensed
aromatic groups, heterocyclic and heteroaromatic groups) group or
moiety with respect to the amino nitrogen; Ar is an arylene group
or moiety (i.e., as understood in the art or as shown and used
herein, e.g., a phenylene, biphenylene, naphthylene, anthrylene,
phenanthrylene, or other single, multiple or polycyclic condensed
aromatic and/or fused heterocyclic moiety); R is a moiety selected
from alkyl, cyclic alkyl, cyclic alkylene, alkenyl, phenyl,
heterocyclic, and heteroaromatic moieties; X and Y are
independently selected from hydrogen, halogen, alkoxide, and amino;
m, m', m'' and m''' are independently integers from 0-5, providing
at least one of m-m''' is from 1-5; n is independently an integer
from 0-3; and o and o' are independently integers selected from
0-5, providing at least one of o and o' is 1-5. With reference to
the preceding description of FIGS. 2A and 2C various combinations
of R, X, Y and n can provide, without limitation, moieties
associated with R.sub.2-R.sub.8 of FIGS. 2A and 2C having 1- about
6 carbon atoms and including dialkyldichlorosilyl,
dialkyldialkoxysilyl, trichlorosilyl and/or trialkoxysilyl
groups.
FIG. 2F shows structure formulae, illustrating various arylamine
compounds in accordance with FIGS. 2A, 2C-E, where R, X, Y and n
are as provided in conjunction with FIGS. 2A, 2C-E.
FIG. 2G illustrates, schematically, synthetic routes 1 and 2 for
preparation of compounds in accordance with FIGS. 2D and 2E,
respectively. R, X, Y and n are as provided above with respect to
FIGS. 2A and 2C-E. Reference is also made to the procedures
illustrated in FIG. 11B and further described in Examples 2 and
27-28.
FIGS. 3A-3C show structural formulae for aryl compounds which are
illustrative of examples of compounds of the type which can be used
as emissive compounds/agents in the preparation of the molecular
conductive layers and electroluminescent media of this invention.
In FIG. 3A, R.sub.9 and R.sub.10 can be H, trihalosilyl,
trialkoxysilyl, dihalosilyl, dialkoxysilyl, or a moiety having 1 to
6 carbon atoms which can include dialkyldichlorosilyl,
dialkyldialkoxysilyl, trichlorosilyl or trialkoxysilyl groups
substituted anywhere on the aryl positions. In FIG. 3B, M is Al or
Ga; and R.sub.11-R.sub.14 can be H, trihalosilyl, trialkoxysilyl,
dihalosilyl, dialkoxysilyl, or a moiety having 1 to 6 carbon atoms
which can include dialkyldichlorosilyl, dialkyldialkoxysilyl,
trichlorosilyl or trialkoxysilyl groups substituted anywhere on the
aryl positions. In FIG. 3C, Ar is arylene; and R.sub.15-R.sub.18
can be H, trihalosilyl, trialkoxysilyl, dihalosilyl, dialkoxysilyl,
or a moiety having 1 to 6 carbon atoms which can include
dialkyldichlorosilyl, dialkyldialkoxysilyl, trichlorosilyl or
trialkoxysilyl groups substituted anywhere on the aryl
positions.
FIGS. 4A-4C show structural formulae for heterocyclic compounds
which are illustrative examples of compounds of the type which can
be used as electron transport components/agents in the preparation
of the molecular conductive or electron transport layers and in
electroluminescent media of this invention. In FIGS. 4A-4C, X is O
or S; and R.sub.19-R.sub.24 can be aryl groups substituted with the
following substituents anywhere on the aryl ring: trihalosilyl,
trialkoxysilyl, dihalosilyl, dialkoxysilyl, or a moiety having 1 to
6 carbon atoms which can contain dialkyldichlorosilyl,
dialkyldialkoxysilyl, trichlorosilyl or trialkoxysilyl groups.
FIGS. 5A and 5B (ITO is indium-tin-oxide; HTL is hole transport
layer and ETL is electron transport layer) show, schematically and
in a step-wise manner by way of illustrating the present invention,
use of the components/agents of Examples 1-5 and FIGS. 1-4 in the
self-assembly and preparation of an organic light-emitting diode
device. In particular, the molecular representation FIG. 5A
illustrates the hydrolysis of an assembled silicon/silane
component/agent to provide an Si--OH functionality reactive toward
a silicon/silane moiety of another component, agent or conductive
layer. The block and molecular representations of FIG. 5B
illustrate a completed assembly.
FIG. 6 shows an alternative synthetic sequence enroute to several
arylamine components/agents, also in accordance with the present
invention.
FIG. 7 shows, schematically and by way of illustrating an
alternative embodiment of the present invention, use of the
components/agents of FIG. 6 in the preparation of another
representative electroluminescent device.
FIG. 8 graphically correlates x-ray reflectivity measurements of
film thickness with the number of capping layers applied to a
substrate. As calculated from the slope of the line (y=8.3184x),
each layer is about 7.84 .ANG. in dimensional thickness.
FIG. 9 graphically shows cyclic voltametry measurements, using
10.sup.-3M ferrocene in acetonitrile, taken after successive layer
(c-e) deposition and as compared to a bare ITO electrode (a). Even
one capping layer (b), in accordance with this invention,
effectively blocks the electrode surface. Complete blocking is
observed after deposition of three or four layers. The sweep rate
was 100 mV/sec, and the electrode area was about 0.7 cm.sup.2.
FIGS. 10A-C graphically illustrate various utilities and/or
performance characteristics (current density, quantum efficiency
and forward light output, respectively, versus voltage) achievable
through use of the present invention, as a function of the number
of capping layers on an electrode surface. 0 layers, bare ITO (), 1
layer, 8 .ANG. (.quadrature.), 2 layers, 17 .ANG. (.cndot.), 3
layers, 25 .ANG. (.tangle-solidup.) and 4 layers, 33 .ANG.
(.gradient.). Reference is made to example 10.
FIG. 11A shows molecular structures of hole adhesion/injection
molecular components: a silyl-functionalized TAA compound (i.e.,
TAA-Si.sub.3, shown after cross-linking), TPD-Si.sub.2 (shown after
crosslinking), and prior art copper phthalocyanine, Cu(Pc). FIG.
11B illustrates one possible scheme for the synthesis of a
preferred TPD-Si.sub.2 adhesion/injection interlayer molecular
precursor. Reference is also made to the procedures described in
Example 2.
FIG. 11C provides structural formulae, with designations described
elsewhere herein, in accordance with the compounds of FIGS. 2A,
2C-E. Reference is made to the synthetic procedures of Examples
27-28 and the comparative studies of Examples 29-38.
FIG. 11D illustrates schemes for the synthesis of two compounds of
FIG. 11C, such schemes analogous to the syntheses described
elsewhere herein, in particular in Example 2 and FIGS. 2G and
11B.
FIGS. 12A-D provide optical microscopic images of vapor-deposited
TPD film (100 nm) morphology after annealing at 80.degree. C. for
1.0 h on ITO substrates coated with a cured 40 nm thick
TPD-Si.sub.2 film (12A) and on bare ITO (12B); polarized optical
image of TPD film (100 nm) morphology before (12C) and after (12D)
annealing the bilayer structure: ITO/CuPc(10 nm)/TPD (100 nm) at
80.degree. C. for 0.50 h.
FIGS. 13A-C show, in turn, (A) light output, (B) external quantum
efficiency, and (C) current-voltage characteristics as a function
of operating voltage for OLED devices having the structure:
ITO/(adhesion/injection/molecular component interlayer)/TPD hole
transport layer (50 nm)/Alq (60 nm)/Al, where the
injection/adhesion component interlayer is prior art Cu(Pc) (10
nm), TAA (15 nm), and TPD-Si.sub.2 (40 nm).
FIG. 14 compares injection characteristics of hole-only devices
having the structure ITO/molecular component interlayer/TPD (250
nm)/Au(5 nm)/Al (180 nm) for various anode functionalization
layers.
FIG. 15 graphically illustrates by comparison the effect of thermal
stressing (90.degree. C. under vacuum) on device characteristics of
ITO/(injection/adhesion interlayer)/TPD(50 nm)/Alq(60 nm)/Al (100
nm) devices, where the molecular component interlayer is
TPD-Si.sub.2 (40 nm), TAA (15 nm), and prior art CuPc (10 nm).
SUMMARY OF THE INVENTION
In light of the foregoing, it is an object of the present invention
to provide electroluminescent articles and/or devices and method(s)
for their production and/or assembly, thereby overcoming various
deficiencies and shortcomings of the prior art, including those
outlined above. It will be understood by those skilled in the art
that one or more aspects of this invention can meet certain
objectives, while one or more other aspects can meet certain other
objectives. Each objective may not apply equally, in all its
respects, to every aspect of this invention. As such, the following
objects can be viewed the alternative with respect to any one
aspect of this invention.
It is an object of the present invention to provide control over
the thickness dimension of a luminescent medium and/or the
conductive layers of such a medium, to control the wavelength of
light emitted from any electroluminescent device and enhance the
efficiency of such emission.
It can be another object of the present invention to provide
molecular components for the construction and/or modification of an
electroluminescent medium and/or the conductive layers thereof,
which will allow lower driving and/or turn-on voltages than are
available through use of conventional materials.
It can also be an object of the present invention to provide
component molecules which can be used effectively in liquid media
without resort to high vacuum or MBE growth equipment.
It can also be an object of the present invention to provide
conformal conductive layers and the molecular components thereof
which allows for the smooth, uniform deposition on an electrode,
substrate surface and/or previously-deposited layers.
It can also be an object of this invention to provide an
electroluminescent medium having a hybrid structure and where one
or more of the layers is applied by a spin-coat or vapor deposition
technique to one or more self-assembled conductive layers.
Other objects, features and advantages of the present invention
will be apparent from this summary of the invention and its
descriptions of various preferred embodiments, and will be readily
apparent to those skilled in the art having knowledge of various
electroluminescent devices and assembly/production techniques. Such
objects, features, benefits and advantages will be apparent from
the above as taken into conjunction with the accompanying examples,
data, figures and all reasonable inferences to be drawn therefrom,
alone or with consideration of the references incorporated
herein.
This invention describes, in part, a new route to the fabrication
of light-emitting organic multilayer heterojunction devices, useful
for both large and small, multicolored display applications. As
described more fully below, electron and hole transporting layers,
as well as the emissive layer, as well as any other additional
layers, are applied, developed and/or modified by molecular
self-assembly techniques. As such, the invention can provide
precise control over the thickness of a luminescent medium or the
conductive layers which make up such a medium, as well as provide
maximum light generation efficiency. Use of the present invention
provides strong covalent bonds between the constituent molecular
components, such that the mechanical, thermal, chemical and/or
photochemical stability of such media and/or conductive layers, as
can be used with an electroluminescent device, are enhanced. The
use of such components also promotes conformal surface coverage to
prevent cracks and pinhole deformities.
More specifically, the siloxane self-assembly techniques described
herein allow for the construction of molecule-based
electroluminescent media and devices. As described more fully
below, various molecular components can be utilized to control the
thickness dimension of the luminescent media and/or conductive
layers. Nanometer dimensions can be obtained, with self-sealing,
conformal coverage. The resulting covalent, hydrophobic siloxane
network imparts considerable mechanical strength, as well as
enhancing the resistance of such media and/or devices to dielectric
breakdown, moisture intrusion, and other degradative processes.
In part, the present invention is an electroluminescent article or
device which includes (1) an anode, (2) a plurality of molecular
conductive layers where one of the layers is coupled to the anode
with silicon-oxygen bonds and each of the layers is coupled one to
another with silicon-oxygen bonds, and (3) a cathode in the
electrical contact with the conductive layers. More generally and
within the scope of this invention, an anode is separated from a
cathode by an organic luminescent medium. The anode and the cathode
are connected to an external power source by conductors. The power
source can be a continuous direct, alternating or an intermittent
current voltage source. A convenient conventional power source,
including any desired switching circuitry, which is capable of
positively biasing the anode with respect to the cathode, can be
employed. Either the anode or cathode can be at ground
potential.
The conductive layers can include but are limited to a hole
transport layer, a hole injection layer, an electron transport
layer and an emissive layer. Under forward biasing conditions, the
anode is at a higher potential than the cathode, and the anode
injects holes (positive charge carriers) into the conductive layers
and/or luminescent medium while the cathode injects electrons
therein. The portion of the layers/medium adjacent to the anode
forms a hole injecting and/or transporting zone while the portion
of the layers/medium adjacent to the cathode forms an electron
injecting and/or transporting zone. The injected holes and
electrons each migrate toward the oppositely charged electrode,
resulting in hole-electron interaction within the organic
luminescent medium of conductive layers. A migrating electron drops
from its conduction potential to a valence band in filling a hole
to release energy as light. In such a manner, the organic
luminescent layers/medium between the electrodes performs as a
luminescent zone receiving mobile charge carriers from each
electrode. Depending upon the construction of the article/device,
the released light can be emitted from the luminescent conductive
layers/medium through one or more of edges separating the
electrodes, through the anode, through the cathode, or through any
combination thereof. See, U.S. Pat. No. 5,409,783 and, in
particular cols. 4-6 and FIG. 1 thereof, which is incorporated
herein by reference in its entirety. As would be understood by
those skilled in the art, reverse biasing of the electrodes will
reverse the direction of mobile charge migration, interrupt charge
injection, and terminate light emission. Consistent with the prior
art, the present invention contemplates a forward biasing DC power
source and reliance on external current interruption or modulation
to regulate light emission.
As demonstrated and explained below, it is possible to maintain a
current density compatible with efficient light emission while
employing a relatively low voltage across the electrodes by
limiting the total thickness of the organic luminescent medium to
nanometer dimensions. At the molecular dimensions possible through
use of this invention, an applied voltage of less than about 10
volts is sufficient for efficient light emission. As discussed more
thoroughly herein, the thickness of the organic luminescent
conductive layers/medium can be designed to control and/or
determine the wavelength of emitted light, as well as reduce the
applied voltage and/or increase in the field potential.
Given the nanometer dimensions of the organic luminescent
layers/medium, light is usually emitted through one of the two
electrodes. The electrode can be formed as a translucent or
transparent coating, either on the organic layer/medium or on a
separate translucent or transparent support. The layer/medium
thickness is constructed to balance light transmission (or
extinction) and electrical conductance (or resistance). Other
considerations relating to the design, construction and/or
structure of such articles or devices are as provided in the above
referenced U.S. Pat. No. 5,409,783, such considerations as would be
modified in accordance with the molecular conductive layers and
assembly methods of the present invention.
In preferred embodiments, the conductive layers have molecular
components, and each molecular component has at least two silicon
moieties. In highly preferred embodiments, each silicon moiety is a
halogenated or alkoxylated silane and silicon-oxygen bonds are
obtainable from the condensation of the silane moieties with
hydroxy functionalities. In preferred embodiments, the present
invention employs an anode with a substrate having a hydroxylated
surface portion. The surface portion is transparent to near-IR and
visible wavelengths of light. In such highly preferred embodiments
the hydroxylated surface portions include SiO.sub.2,
In.sub.2O.sub.3.xSnO.sub.2, Ge and Si, among other such
materials.
In conjunction with anodes and the hydroxylated surface portions
thereof, the conductive layers include molecular components, and
each molecular component has at least two silicon moieties. As
discussed above, in such embodiments, each silicon moiety is a
halogenated or alkoxylated silane, and silicon-oxygen bonds are
obtainable from the condensation of the silane moieties with
hydroxy functionalities which can be on a surface portion of an
anode. Consistent with such preferred embodiments, a cathode is in
electrical contact with the conductive layers. In highly preferred
embodiments, the cathode is vapor deposited on the conductive
layers, and constructed of a material including Al, Mg, Ag, Au, In,
Ca and alloys thereof.
In part, the present invention is a method of producing a
light-emitting diode having enhanced stability and light generation
efficiency. The method includes (1) providing an anode with a
hydroxylated surface; (2) coupling the surface to a hole transport
layer having a plurality of molecular components, with each
component having at least two silicon moieties reactive with the
surface, with coupling of one of the silicon moieties to form
silicon-oxygen bonds between the surface and the hole transport
layer; (3) coupling the hole transport layer to an electron
transport layer, the electron transport layer having a plurality of
molecular components with each of the components having at least
two silicon moieties reactive with the hole transport layer, with
the coupling of one of the silicon moieties to form silicon-oxygen
bonds between the hole and electron transport layers; and (4)
contacting the electron transport layer with a cathode
material.
In preferred embodiments of this method, the hole transport layer
includes a hole injecting zone of molecular components and a hole
transporting zone of molecular components. Likewise, in preferred
embodiments, each silicon moiety is a halogenated or alkoxylated
silane such that, with respect to this embodiment, coupling the
hole transport layer to the electron transport layer further
includes hydrolyzing the halogenated or alkoxylated silane.
Likewise, with respect to a halogenated or alkoxylated silane
embodiment, contacting the electron transport layer with the
cathode further includes hydrolyzing the silane.
In part, the present invention is a method of controlling the
wavelength of light emitted from an electroluminescent device. The
inventive method includes (1) providing in sequence a hole
transport layer, an emissive layer and an electron transport layer
to form a medium of organic luminescent layers; and (2) modifying
the thickness dimension of at least one of the layers, each of the
layers including molecular components corresponding to the layer
and having at least two silicon moieties reactive to a hydroxy
functionality and the layers coupled one to another by Si--O bonds,
the modification by reaction of the corresponding molecular
components one to another to form Si--O bonds between the molecular
components, and the modification in sequence of the provision of
the layers.
In preferred embodiments of this inventive method, at least one
silicon moiety is unreacted after reaction with a hydroxy
functionality. In highly preferred embodiments, modification then
includes hydrolyzing the unreacted silicon moiety of one of the
molecular components to form a hydroxysilyl functionality and
condensing the hydroxysilyl functionality with a silicon moiety of
another molecular component to form a siloxane bond sequence
between the molecular components.
In highly preferred embodiments, the silicon moieties are
halogenated or alkoxylated silane moieties. Such embodiments
include modifying the thickness dimension by hydrolyzing the
unreacted silane moiety of one of the molecular components to form
a hydroxysilyl functionality and condensing the hydroxysilyl
functionality with a silane moiety of another molecular component
to form a siloxane bond sequence between the molecular
components.
While the organic luminescent conductive layers/medium of this
invention can be described as having a single organic hole
injecting or transporting layer and a single electron injecting or
transporting layer, modification of each of these layers with
respect to dimensional thickness or into multiple layers, as more
specifically described below, can result in further refinement or
enhancement of device performance by way of the light emitted
therefrom. When multiple electron injecting and transporting layers
are present, the layer receiving holes is the layer in which
hole-electron interaction occurs, thereby forming the luminescent
or emissive layer of the device.
The articles/devices of this invention can emit light through
either the cathode or the anode. Where emission is through the
cathode, the anode need not be light transmissive. Transparent
anodes can be formed of selected metal oxides or a combination of
metal oxides having a suitably high work function. Preferred metal
oxides have a work function of greater than 4 electron volts (eV).
Suitable anode metal oxides can be chosen from among the high
(>4 eV) work function materials. A transparent anode can also be
formed of a transparent metal oxide layer on a support or as a
separate foil or sheet.
The devices/articles of this invention can employ a cathode
constructed of any metal, including any high or low work function
metal, heretofore taught to be useful for this purpose and as
further elaborated in that portion of the incorporated patent
referenced in the preceding paragraph. As mentioned therein,
fabrication, performance, and stability advantages can be realized
by forming the cathode of a combination of a low work function
(<4 eV) metal and at least one other metal. Available low work
function metal choices for the cathode are listed in cols. 19-20 of
the aforementioned incorporated patent, by periods of the Periodic
Table of Elements and categorized into 0.5 eV work function groups.
All work functions provided therein are from Sze, Physics of
Semiconductor Devices, Wiley, N.Y., 1969, p. 366.
A second metal can be included in the cathode to increase storage
and operational stability. The second metal can be chosen from
among any metal other than an alkali metal. The second metal can
itself be a low work function metal and thus be chosen from the
above-referenced list and having a work function of less than 4 eV.
To the extent that the second metal exhibits a low work function it
can, of course, supplement the first metal in facilitating electron
injection.
Alternatively, the second metal can be chosen from any of the
various metals having a work function greater than 4 eV. These
metals include elements resistant to oxidation and, therefore,
those more commonly fabricated as metallic elements. To the extent
the second metal remains invariant in the article or device, it can
contribute to the stability. Available higher work function (4 eV
or greater) metal choices for the cathode are listed in lines 50-69
of col. 20 and lines 1-15 of col. 21 of the aforementioned
incorporated patent, by periods of the Periodic Table of Elements
and categorized into 0.5 eV work function groups.
As described more fully in U.S. Pat. No. 5,156,918 which is
incorporated herein by reference in its entirety, the electrodes
and/or substrates of this invention have, preferably, a surface
with polar reactive groups, such as a hydroxyl (--OH) group.
Materials suitable for use with or as electrodes and/or substrates
for anchoring the conductive layers and luminescent media of this
invention should conform to the following requirements: any solid
material exposing a high energy (polar) surface to which
layer-forming molecules can bind. These may include: metals, metal
oxides such as SiO.sub.2, TiO.sub.2, MgO, and Al.sub.2O.sub.3
(sapphire), semiconductors, glasses, silica, quartz, salts, organic
and inorganic polymers, organic and inorganic crystals and the
like.
Inorganic oxides (in the form of crystals or thin films) are
especially preferred because oxides yield satisfactory hydrophilic
metal hydroxyl groups on the surface upon proper treatment. These
hydroxyl groups react readily with a variety of silyl coupling
reagents to introduce desired coupling functionalities that can in
turn facilitate the introduction of other organic components.
The physical and chemical nature of the anode materials dictates
specific cleaning procedures to improve the utility of this
invention. Alkaline processes (NaOH aq.) are generally used. This
process will generate a fresh hydroxylated surface layer on the
substrates while the metal oxide bond on the surface is cleaved to
form vicinal hydroxyl groups. High surface hydroxyl densities on
the anode surface can be obtained by sonicating the substrates in
an aqueous base bath. The hydroxyl groups on the surface will
anchor and orient any of the molecular components/agents described
herein. As described more fully below, molecules such as
organosilanes with hydrophilic functional groups can orient to form
the conductive layers.
Other considerations relating to the design, material choice and
construction of electrodes and/or substrates useful with this
invention are as provided in the above referenced and incorporated
U.S. Pat. No. 5,409,783 and in particular cols. 21-23 thereof, such
considerations as would be modified by those skilled in the art in
accordance with the molecular conductive layers, and assembly
methods and objects of the present invention.
The conductive layers and/or organic luminescent medium of the
devices/articles of this invention preferably contain at least two
separate layers, at least one layer for transporting electrons
injected from the cathode and at least one layer for transporting
holes injected from the anode. As is more specifically taught in
U.S. Pat. No. 4,720,432, incorporated herein by reference in its
entirety, the latter is in turn preferably at least two layers, one
in contact with the anode, providing a hole injecting zone and a
layer between the hole injecting zone and the electron transport
layer, providing a hole transporting zone. While several preferred
embodiments of this invention are described as employing at least
three separate organic layers, it will be appreciated that either
the layer forming the hole injecting zone or the layer forming the
hole transporting zone can be omitted and the remaining layer will
perform both functions. However, enhanced initial and sustained
performance levels of the articles or devices of this invention can
be realized when separate hole injecting and hole transporting
layers are used in combination.
Porphyrinic and phthalocyanic compounds of the type described in
cols. 11-15 of the referenced/incorporated U.S. Pat. No. 5,409,783
can be used to form the hole injecting zone. In particular, the
phthalocyanine structure shown in column 11 is representative,
particularly where X can be, but is not limited to, an
alkyltrichlorosilane, alkyltrialkoxysilane, dialkyldialkoxysilane,
or dialkyldichlorosilane functionality and where the alkyl and
alkoxy groups can contain 1-6 carbon atoms or is hydrogen.
Preferred porphyrinic compounds are represented by the structure
shown in col. 14 and where R, T.sup.1 and T.sup.2 can be but are
not limited to an alkyltrichlorosilane, alkyltrialkoxysilane,
dialkyldialkoxysilane, or dialkyldichlorosilane functionality and
where the alkyl and alkoxy groups contain 1-6 carbon atoms or is
hydrogen. (See, also, FIGS. 1A and 1B, herein.) Preferred
phthalocyanine- and porphyrin-based hole injection agents include
silicon phthalocyanine dichloride and
5,10,15,20-tetraphenyl-21H,23H-porphine silicon (IV) dichloride,
respectively.
The hole transporting layer is preferably one which contains at
least one tertiary aromatic amine, examples of which are as
described in FIGS. 2A-2F and examples 1-2, 19-22 and 28a-28g. Such
layers can comprise, without limitation, compounds of the sort
provided in FIGS. 2A, 2C-F, where at least one of the aromatic
moieties (i.e., phenyl in FIGS. 2A and 2C, and one of Ar.sub.1-4 in
FIGS. 2D-F) is substituted with at least one pendant silane moiety
comprising a hydrolyzable silyl group (e.g., halo, alkoxy, etc.).
Other exemplary arylamine core structures are illustrated in U.S.
Pat. No. 3,180,730, which is incorporated herein by reference in
its entirety, where the core structures are modified as described
herein. Other suitable triarylamines substituted with a vinyl or
vinylene radical and/or containing at least one active hydrogen
containing group are disclosed in U.S. Pat. Nos. 5,409,783,
3,567,450 and 3,658,520. These patents are incorporated herein by
reference in their entirety and the core structures disclosed are
modified as described herein. In particular, with respect to the
arylamines represented by structural formulas XXI and XXIII in
cols. 15-16 of U.S. Pat. No. 5,409,703, R.sup.24, R.sup.25,
R.sup.26, R.sup.27, R.sup.30, R.sup.31 and R.sup.32 can be an
alkyltrichlorosilane, alkyltrialkoxysilane, dialkyldialkoxysilane,
or dialkyldichlorosilane functionality where the alkyl and alkoxy
groups can contain 1- about 6 carbon atoms or is hydrogen.
Molecular components of this invention comprising emissive agents
and/or the emissive layer include those described herein in FIGS.
3A-3C and Example 5. Other such components/agents include various
metal chelated oxinoid compounds, including chelates of oxine (also
commonly referred to as 8-quinolinol or 8-hydroxyquinoline), such
as those represented by structure III in col. 8 of the referenced
and incorporated U.S. Pat. No. 5,409,783, and where Z.sup.2 can be
but is not limited to an alkyltrichlorosilane,
alkyltrialkoxysilane, dialkyldialkoxysilane, or
dialkyldichlorosilane functionality and where the alkyl and alkoxy
groups can contain 1-6 carbon atoms or is hydrogen. Other such
molecular components/emissive agents include the quinolinolato
compounds represented in cols. 7-8 of U.S. Pat. No. 5,151,629, also
incorporated herein by reference in its entirety, where a ring
substituent can be but is not limited to an alkyltrichlorosilane,
alkyltrialkoxysilane, dialkyldialkoxysilane, or
dialkyldichlorosilane functionality and where the alkyl and alkoxy
groups can contain 1-6 carbon atoms or is hydrogen. In a similar
fashion, the dimethylidene compounds of U.S. Pat. No. 5,130,603,
also incorporated herein by reference in its entirety, can be used,
as modified in accordance with this invention such that the aryl
substituents can include an alkyltrichlorosilane,
alkyltrialkoxysilane, dialkyldialkoxysilane, or
dialkyldichlorosilane functionality and where the alkyl and alkoxy
groups can contain 1-6 carbon atoms or is hydrogen.
Other components which can be used as emissive agents include
without limitation anthracene, naphthalene, phenanthrene, pyrene,
chrysene, perylene and other fused ring organic or metal-organic
compounds, or as provided in col. 17 of the previously referenced
and incorporated U.S. Pat. No. 5,409,783, such compounds as
modified in accordance with this invention and as more fully
described above. Modifiable components also include those described
in U.S. Pat. Nos. 3,172,862, 3,173,050 and 3,710,167--all of which
are incorporated herein by reference in their entirety.
Molecular components which can be utilized as electron injecting or
electron transport agents and/or in conjunction with an electron
injection or electron transport layer are as described in FIGS.
4A-4C and Examples 3(a)-(d) and 4. Other such components include
oxadiazole compounds such as those shown in cols. 12-13 of U.S.
Pat. No. 5,276,381, also incorporated herein by reference in its
entirety, as such compounds would be modified in accordance with
this invention such that the phenyl substituents thereof each
include an alkyltrichlorosilane, alkyltrialkoxysilane,
dialkyldialkoxysilane, or dialkyldichlorosilane functionality and
where the alkyl and alkoxy groups can contain 1-6 carbon atoms or
is hydrogen. Likewise, such components can be derived from the
thiadiazole compounds described in U.S. Pat. No. 5,336,546 which is
incorporated herein by reference in its entirety.
As described above, inorganic silicon moieties can be used in
conjunction with the various molecular components, agents,
conductive layers and/or capping layers. In particular, silane
moieties can be used with good effect to impart mechanical,
thermal, chemical and/or photochemical stability to the luminescent
medium and/or device. Such moieties are especially useful in
conjunction with the methodology described herein. Degradation is
minimized until further synthetic modification is desired.
Hydrolysis of an unreacted silicon/silane moiety provides an Si--OH
functionality reactive with a silicon/silane moiety of another
component, agent and/or conductive layer. Hydrolysis proceeds
quickly in quantitative yield, as does a subsequent condensation
reaction with an unreacted silicon/silane moiety of another
component to provide a siloxane bond sequence between components,
agents and/or conductive layers.
In general, the molecular agents/components in FIGS. 1-4 can be
prepared with a lithium or Grignard reagent using synthetic
techniques known to one skilled in the art and subsequent reaction
with halosilane or alkoxysilane reagents. Alternatively,
unsaturated olefinic or acetylenic groups can be appended from the
core structures using known synthetic techniques. Subsequently,
halosilane or alkoxysilane functional groups can be introduced
using hydrosilation techniques, also known to one skilled in the
art. For instance, reference is made to the synthetic schemes
described in Examples 2-3 and 28a-g and FIGS. 11B and 11D. Such
substitution, lithiation, allylation and/or silylation techniques
are as would be understood by those skilled in the art in
conjunction with the schematic illustration of FIG. 2G.
Purification is carried out using procedures appropriate for the
specific target molecule.
It has been observed previously that the performance
characteristics of electroluminescent articles of the type
described herein can be enhanced by the incorporation of a layer
having a modifying function between the cathode and, for instance,
an electron transport or emissive layer. Previous studies show that
the vapor deposition of thin layers of LiF into the various
emissive and electron transport layers before deposition of the
cathode improves performance in the areas of luminescence and
quantum efficiency. However, this technique is limited in that the
deposited LiF films are rough, degrade in air and do not form
comformal, pinhole-free coatings.
The present invention is also directed to the application of
self-assembly techniques to form layers which cap an electrode,
provide dielectric and other functions and/or enhance performance
relative to the prior art. Such capping layers are self-assembled
films which are conformal in their coverage, can have dimensions
less than one nanometer and can be deposited with a great deal of
control over the total layer thickness. Accordingly, the present
invention also includes an electroluminescent article or device
which includes (1) an anode, (2) at least one molecular capping
layer coupled to the anode with silicon-oxygen bonds, with each
capping layer coupled one to another with silicon-oxygen bonds, (3)
a plurality of molecular conductive layers, with one of the layers
coupled to the capping layer with silicon-oxygen bonds and each
conductive layer coupled one to another with silicon-oxygen bonds,
and (4) a cathode in electric contact with a conductive layer.
Likewise, and in accordance with this invention, the capping layer
can be deposited on a conductive layer and/or otherwise introduced
so as to be adjacent to a cathode, to enhance overall
performance.
More generally and within the scope of this invention, the anode is
separated from the cathode by an organic luminescent medium. The
anode and cathode are connected to an external power source by
conductors. The power source can be a continuous direct,
alternating or intermittent current voltage source. A convenient
conventional power source, including any desired switching
circuitry, which is capable of positively biasing the anode with
respect to the cathode, can be employed. Either the anode or
cathode can be at ground potential.
In preferred embodiments, each conductive and/or capping layer has
molecular components, and each molecular component has at least two
silicon moieties. In highly preferred embodiments, each such
conductive and/or capping component is a halogenated or alkoxylated
silane, and silicon-oxygen bonds are obtainable from the
condensation of the silane moieties with hydroxy functionalities.
Without limitation, a preferred capping material is
octachlorotrisiloxane. The anode and cathode can be chosen and/or
constructed as otherwise described herein.
In part, the present invention is a method of using molecular
dimension to control the forward light output of an
electroluminescent device. The inventive method includes (1)
providing an electrode and a molecular layer thereon, the layer
coupled to the electrode with first molecular components having at
least two silicon moieties reactive to a hydroxy functionality; and
(2) modifying the thickness of the layer by reacting the molecular
components with second components to form a siloxane bond sequence
between the first and second molecular components, the second
molecular components having at least two silicon moieties also
reactive to a hydroxy functionality.
In preferred embodiments of this inventive method, at least one
silicon moiety is unreacted after reaction with a hydroxy
functionality. In highly preferred embodiments, the modification
further includes hydrolyzing an unreacted silicon moiety of one of
the molecular components to form a hydroxysilyl functionality and
condensing the hydroxysilyl functionality with a silicon moiety of
a third molecular component to form a siloxane bond sequence
between the second and third molecular components. In highly
preferred embodiments, the silicon moieties are halogenated or
alkoxylated silane moieties.
In part, the present invention also includes any electroluminescent
article for generating light upon application of an electrical
potential across two electrodes. Such an article includes an
electrode having a surface portion and a molecular layer coupled
and/or capped thereon. The layer includes molecular components, and
each component has at least two silicon moieties. The layer is
coupled to the electrode with silicon-oxygen bonds. In preferred
embodiments, each silicon moiety is a halogenated silane, and
silicon-oxygen bonds are obtained from a condensation reaction.
Likewise, and without limitation, the electrode has a substrate
with a hydroxylated surface portion transparent to near-IR and
visible wavelengths of light. Such a layer can be utilized to cap
the electrode and/or enhance performance as otherwise described
herein. More generally, in such an article or any other described
herein, the luminescent medium can be constructed using either the
self-assembly techniques described herein or the materials and
techniques of the prior art.
The electroluminescent devices and related methods of this
invention can demonstrate various interlayer/interfacial phenomena
through choice of layer/molecular components and design of the
resulting electroluminescent medium. As a point of reference, a
number of cathode and anode interfacial structures can enhance
charge injection, hence device performance. For instance, with
vapor-deposited, anode/TPD
(N--N'-diphenyl-N--N'-bis(3-methylphenyl)-(1-1'-biphenyl)-4-4'-diamine)/A-
lq (tris(quinoxalinato)Al(III))/cathode devices of the prior art, a
dramatic increase in light output and quantum efficiency occurs
when .ANG.-scale LiF or CsF layers are interposed between the
cathode and electron transport layer (ETL). Such thin dielectric
layers are thought to lower the Al work function, thus reducing the
effective electron injection barrier (energy level offset between
the Alq LUMO and the Al Fermi level).
In contrast, modification of the ITO anode--hole transport layer
(HTL) interface is somewhat more controllable, although similar
mechanistic uncertainties pertain. Thus, a variety of ITO
functionalization approaches produce phenomenologically similar
effects, although less dramatic than those observed for the
interposition of alkali fluoride at Al cathodes. These approaches
include deposition onto ITO of nanoscale layers of various organic
acids, copper phthalocyanine, or thicker (30-100 nm) layers of
polyaniline or polythiophene (PEDOT), all resulting in somewhat
enhanced luminous performance. Explanations for these phenomena are
diverse, ranging from altering interfacial electric fields,
balancing electron/hole injection fluence, confining electrons in
the emissive layer, reducing injected charge back-scattering, and
moderating anode Fermi level-HTL HOMO energetic discontinuities.
This diversity of proposed mechanisms accurately reflects the
complexity of interactions at OLED interfaces and, in many cases,
the lack of necessary microstructural information.
In a departure from the prior art, the present invention can be
considered in the context of one or more structural relationships
between an OLED anode and/or its associated organic layers. Without
restriction to any one theory or mode of operation, moderation of
the surface energy mismatch can be effected at a hydrophilic oxide
anode-hydrophobic HTL interface, as demonstrated below using
nanoscopic self-assembled silyl group functionalized amine
components (see, for example, FIG. 11A and 1-4 TAA layers; 11
.ANG./layer). Promoting anode/ITO-HTL physical cohesion
significantly enhances luminous performance and durability.
Furthermore, as relates to another aspect of this invention, a
silyl-group functionalized, crosslinkable amine layer having the
core amine structure of the HTL component, TPD, (TPD-Si.sub.2, FIG.
11A) significantly improves ITO/TPD/Alq/Al device performance and
thermal durability (one metric of device stability) to an extent
surprising, unexpected and unattainable with other anode
functionalization structures. In contrast thereto, the commonly
used copper phthalocyanine (Cu(Pc); FIG. 11A) anode
functionalization layer actually templates crystallization of
overlying TPD films at modest temperatures (Example 13 and FIG.
12D), consistent with the thermal instability of many
Cu(Pc)-buffered OLED devices of the prior art.
Accordingly, in its broader respects, the present invention
contemplates a method of using an amine molecular component to
enhance hole injection across the electrode-organic interface of a
light emitting diode device. The inventive method includes (1)
providing an anode; and (2) incorporating an electroluminescent
medium adjacent the anode, the medium including but not limited to
a molecular layer, coupled to the anode, of amine molecular
components substituted with at least one silyl group, and thereon a
hole transport layer of molecular components having the amine
structure of the aforementioned molecular layer components. The
molecular layer can have at least one of an arylamine component and
an arylalkylamine component, including but not limited to those
monoarylamine, diarylamine and triarylamine components described in
the aforementioned and incorporated U.S. Pat. No. 5,409,783,
modified and/or silyl-functionalized as provided herein. Other
suitable arylamine and/or arylalkylamine structures are disclosed
in U.S. Pat. Nos. 3,180,730, 3,567,450 and 3,658,520, each of which
is incorporated herein in its entirety, such structures as can also
be modified to provide silyl-functionality in accordance herewith.
Likewise, a combination of such silyl-substituted components can be
employed with beneficial effect.
In preferred embodiments of this inventive method, the
aforementioned amine molecular layer components are
alkylsilyl-substituted compounds of the type illustrated in FIGS.
2A and 2C-F. In highly preferred embodiments, such components
include the alkylsilyl-substituted TAA and alkylsilyl substituted
TPD compounds prepared as described herein. Regardless, such a
molecular layer can be spin-coated on the anode surface or
self-assembled, as described more fully above, to provide
silicon-oxygen bonds therewith. A plurality of such molecular
layers can be coupled successively on an anode surface--each layer
coupled one to another with silicon-oxygen bonds--to improve
structural stability and enhance device performance. As described
herein and with reference to several of the following examples,
hole injection can be enhanced by choice of a molecular layer with
components having a structural relationship with those arylamine or
arylalkylamine components of the hole transport layer. In preferred
embodiments, such enhancement can be achieved through use of a
silyl-functionalized TPD layer in conjunction with a TPD hole
transport layer.
As such, the present invention also includes an organic
electroluminescent device for generating light upon application of
an electrical potential cross to electrodes. Such a device includes
(1) an anode; (2) at least one molecular layer, coupled to the
anode, of one or more of the aforementioned amine molecular
components substituted with at least one silyl group; (3) a
conductive layer of molecular components having the amine
structure; and (4) a cathode in electrical contact with the anode.
A preferred conductive layer includes a hole transport layer
comprising components of the prior art incorporated herein by
reference, or modified as described above. Preferred molecular
layer components are alkylsilyl-substituted compounds of the type
illustrated in FIGS. 2A and 2C-F, in particular
silyl-functionalized TAA and TPD. In light of the aforementioned
structural relationships and associated methodologies, a preferred
conductive layer of such a device is a TPD hole transport layer,
such a layer substantially without crystallization upon annealing
and/or at device operation temperatures when used in conjunction
with a molecular layer of components having the same or a
structurally similar amine structure.
As demonstrated herein, hydrophobic amine HTL--hydrophilic anode
integrity is a factor in OLED performance; poor physical cohesion
contributes to inefficient hole injection and ultimately, device
failure. Enhanced performance can be achieved through use of
molecular layer structures which maximize interfacial cohesion and
charge transport. With reference to one preferred embodiment, a
conveniently applied, spincoated silyl (Si) functionalized TPD
analogue, TPD-Si.sub.2 (FIGS. 11A-B), structurally similar to the
overlying HTL, hence well-suited to stabilizing the interface,
undergoes rapid crosslinking upon spincoating from solution and
subsequent thermal curing to form a dense, robust siloxane matrix
with imbedded TPD hole-transport components. The thickness of these
layers (.about.40 nm) was determined by specular X-ray reflectivity
on samples deposited via identical techniques on single-crystal
silicon. The RMS roughness of the TPD-Si.sub.2 molecular layer
films on ITO substrates of 30 .ANG. RMS roughness is 8-12 .ANG. by
contact mode AFM. Crosslinked TPD-Si.sub.2 films exhibit high
thermal stability, with only 5% weight loss observed up to
400.degree. C. by TGA, indicating substantial resistance to thermal
degradation. Furthermore, cyclic voltammetry of 40 nm TPD-Si.sub.2
films on ITO electrodes indicates that they support facile hole
transport and are electrochemically stable.
The densely crosslinked nature of TPD-Si.sub.2 molecular layer
films is evident in the relatively large separation of oxidative
and reductive peaks (200 mV), suggesting kinetically hindered
oxidation/reduction processes with retarded counterion mobility. P.
E. Smolenyak, E. J. Osburn, S.-Y. Chen, L.-K. Chau, D. F. O'Brian,
N. R. Armstrong, Langmuir 1997, 21, 6568. That TPD-Si.sub.2 film
coverage on ITO is conformal and largely pinhole-free is supported
by studies using a previously described ferrocene probe technique.
W. Li, Q. Wang, J. Cui, H. Chou, T. J. Marks, G. E. Jabbour, S. E.
Shaheen, B. Kippelen, N. Pegyhambarian, P. Dutta, A. J. Richter, J.
Anderson, P. Lee, N. Armstrong, Adv. Mater. 1999, 11, 730. The lack
of significant current flow near the formal potential for ferrocene
oxidation at a TPD-Si.sub.2-coated ITO working electrode indicates
suppression of ferrocene oxidation, consistent with largely
pinhole-free surface coverage. G. Inzelt, Electroanalytical
Chemistry, Vol. 18, Marcel Dekker, New York, 1994, p. 89.
Such silyl-functionalized compounds are further described as having
hole transport-capability in conjunction with high-performance
polymeric light-emitting diodes (PLED), as described in co-pending
international application no. PCT/US03/07963 filed Mar. 14, 2003,
the entirety of which (in particular pages 24-25, 27-29, examples
27-32 and FIGS. 16-19) is incorporated herein by reference. As
discussed therein, device structures having enhanced light emission
can be fabricated without resort to a polymeric HTL. In such
embodiments, as described elsewhere herein, a monolayer of any of
the present silyl-functionalized arylamine compounds can be
self-assembled onto an anode component. The resulting smooth,
contiguous and conformal layer provides a siloxane network with
embedded, electroactive arylamine hole-transporting units.
Further, as a variation of the preceding incorporated description,
the present arylamine compounds can be used to fabricate a hole
transport layer for PLED devices, as described in co-pending
application Ser. No. 60/628,325, filed Dec. 10, 2003, the entirety
of which is incorporated herein by reference. The
silyl-functionalized compounds of this invention can be blended
with a suitable hole-transporting/insulating polymer of the prior
art. Spincoating of a blended solution, with curing, provides a
cross-linked arylamine network with embedded polymeric components.
Both the arylamine and polymeric components provide hole transport
function and other benefits of the type described herein.
Regardless of device structure or fabrication, the arylamine
compounds of this invention can be used to effect
electroluminescent performance. For purposes of illustration, a
series of molecules having incrementally varied structures and
surface linking characteristics were prepared--as described for
compounds in accordance with FIGS. 2A-G and 11A-C--and shown to
form conformal, robust, self-assembled monolayers on OLED anodes.
With reference to examples 27-38, it is seen that molecular
structure effects on OLED charge injection, charge transport, and
response characteristics can be significant.
EXAMPLES OF THE INVENTION
The following non-limiting examples and data illustrate various
aspects and features relating to the articles/devices and/or
methods of the present invention, including the assembly of a
luminescent medium having various molecular components/agents
and/or conductive layers, as are available through the synthetic
methodology described herein. In comparison with the prior art, the
present methods and articles/devices provide results and data which
are surprising, unexpected and contrary to the prior art. While the
utility of this invention is illustrated through the use of several
articles/devices and molecular components/agents/layers which can
be used therewith, it will be understood by those skilled in the
art that comparable results are obtainable with various other
articles/devices and components/agents/layers, as are commensurate
with the scope of this invention.
Example 1
##STR00001##
Synthesis of a Silanated Hole Transport Agent [1]. With reference
to reaction scheme, above, hole transport components, agents and/or
layers can be prepared, in accordance with this invention and/or
for use in conjunction with light-emitting diodes and other similar
electroluminescent devices. Accordingly, 500 mg. (1.0 mmole) of
trisbromophenylamine (Aldrich Chemical Company) was dissolved in 30
ml of dry dimethoxyethane (DME). This solution was cooled to
-45.degree. C. and 1.2 ml (3.3 mmole) of a 2.5 M solution of
n-butyl lithium in hexane was added to the reaction mixture. The
entire mixture was then slowly warmed to 20.degree. C. After
stirring at 20.degree. C. for an additional hour, the solvent was
removed in vacuo. The resulting white precipitate was washed
(3.times.20 ml) with dry pentane and redissolved in 30 ml dry DME.
This solution was subsequently poured into 10 ml (87 mmole) of
silicon tetrachloride at a rate of 1 ml/min. The entire reaction
mixture was then refluxed for two hours. The resulting supernatent
was separated from the precipitate, and the solvent again removed
in vacuo yielding a green-brown residue. A white solid was obtained
from this residue upon sublimation at 10.sup.-6 torr.
Characterization: .sup.1H NMR (600 MHz, C.sub.6D.sub.6, 20.degree.
C.): .delta. 7.07 (d, 6H, Ar--H); .delta. 7.05 (d, 6H, Ar--H);
EI-MS (m/z): 645 (M+).
Example 2
With reference to FIGS. 2A-2C and the representative arylamines
provided therein, other hole transport agents and/or layers of this
invention can be obtained by straightforward application of the
silanation procedure described above in Example 1, with routine
synthetic modification(s) and optimization of reaction conditions
as would be well-known to those skilled in the art and as required
by the particular arylamine. Likewise, preliminary
halogenation/bromination can be effected using known synthetic
procedures. Alternatively, the arylamines of FIGS. 2A-2C and other
suitable substrates can be prepared using other available synthetic
procedures to provide multiple silane reaction centers for use with
the self-assembly methods and light-emitting diodes of this
invention. Core molecular substrates of the type from which the
arylamines of FIGS. 2A-2C can be prepared are described by
Strukelji et al. in Science, 267, 1969 (1995), which is
incorporated herein by reference in its entirety.
Example 3
Synthesis of a Silanated Electron Transport Agent. With reference
to Examples 3(a)-(d) and corresponding reaction schemes, below,
electron transport agents and/or layers can be prepared, in
accordance with this invention and/or for use in conjunction with
light-emitting diodes and other similar electroluminescent
devices.
##STR00002##
Example 3a
Synthesis of 4'-bromo-2-(4-bromobenzoyl)acetophenone [2]. In a
1-liter three neck round bottom flask, 43 g (0.2 mol) methyl
4-bromobenzoic acid and 17.6 g (0.4 mol) sodium hydride were
dissolved in 200 ml dried benzene and heated to 60.degree. C. Next,
39.8 g (0.2 mol) 4-bromoacetophenone in 100 ml dry benzene was
slowly added through a dropping funnel, and 1 ml methanol was added
to the flask to initiate the reaction. After the mixture was
refluxed overnight, the reaction was quenched by adding methanol
and pouring it into ice water. The pH of the mixture was brought
down to 7.0 using 5 N sulfuric acid. A solid was collected, washed
with water, and recrystallized from benzene to give a light yellow
product. Characterization. Yield: 30.3 g (40%). .sup.1H NMR (300
MHz, CDCl.sub.3, 20.degree. C., .delta.): 7.84 (d, 4H, ArH); 7.62
(d, 4H, ArH); 6.77 (s, 2H, CH.sub.2). EI-MS: 382 (M+), 301, 225,
183, 157.
Example 3b
Synthesis of 3,5-bis(4-bromophenyl)isoxazole [3]. In a 250 ml round
bottom flask, 4 g (10.4 mmol) of [2] was dissolved in 100 ml dry
dioxane and heated to reflux, then 3.0 g (43.2 mmol) hydroxylamine
hydrogen chloride in 10 ml water and 5 ml (25 mmol) 5 N NaOH was
then dropped into the refluxing mixture. After 12 hours, the
reaction mixture was cooled down to room temperature, and the
solvent was removed in vacuo. The product was recrystallized from
ethanol. Characterization. Yield: 3.41 g (85%). M.P.
218.5-219.5.degree. C. .sup.1H NMR (300 MHz, CDCl.sub.3, 20.degree.
C., .delta.): 7.78 (d, 2H, ArH), 7.74 (d, 2H, Ar'H), 7.66 (d, 2H,
ArH), 7.62 (d, 2H, Ar'H), 6.82 (s, 1H, isoxazole proton). EI-MS:
379 (M+), 224, 183, 155.
Example 3c
Synthesis of 3,5-bis(4-allylphenyl)isoxazole [4]. In a 250 ml
three-neck round bottom flask, 3.77 g (10 mmol) of [3], 460 mg.
(0.4 mmol) tetrakis-(triphenylphosphine)palladium, and 7.28 g (22
mmol) tributylallyltin were dissolved in 100 ml. dried toluene and
degassed with nitrogen for 30 min. The mixture was heated to
100.degree. C. for 10 h, then cooled down to room temperature.
Next, 50 ml. of a saturated aqueous ammonium fluoride solution was
subsequently added to the mixture. The mixture was extracted with
ether, and the combined organic layer was washed by water, then
brine, and finally dried over sodium sulfate. The solvent was
removed in vacuo. The residue was purified by column
chromatography. (first, 100% hexanes, then chloroform:hexanes
[80:20]). Characterization. Yield: 1.55 g (57%). .sup.1H NMR (300
MHz, CDCl.sub.3, 20.degree. C., .delta.): 7.78 (d, 2H, ArH), 7.74
(d, 2H, Ar'H), 7.34 (d, 2H, ArH), 7.30 (d, 2H, Ar'H), 6.78 (s, 1H,
isoxazole proton), 5.96 (m, 2H, alkene H), 5.14 (d, 4H, terminal
alkene H), 3.44 (d, 4H, methylene group). EI-MS: 299 (M+), 258,
217.
Example 3d
Synthesis of 3,5-bis(4-(N-trichlorosilyl)propylphenyl)isoxazole
[5]. To 2 ml of THF was added 5 mg of [4], 3.4 .mu.l of HSiCl.sub.3
and 0.8 mg. of H.sub.2PtCl.sub.6 were added to 2 ml of THF. The
reaction was heated at 50.degree. C. for 14 h. The solvent was then
removed in vacuo. A white solid was obtained from this residue upon
sublimation at 10.sup.-6 torr. Characterization. .sup.1H NMR (300
MHz, d.sup.8-THF, 20.degree. C., .delta.): 7.72 (d, 2H, ArH), 7.68
(d, 2H, Ar'H), 7.36 (d, 2H, ArH), 7.32 (d, 2H, Ar'H), 6.30 (s, 1H,
isoxazole); 2.52 (t, 2H, CH); 1.55 (m, 4H, CH.sub.2); 0.85 (t, 6H,
CH.sub.3).
Example 4
With reference to FIGS. 4A-4C and the representative heterocycles
provided-therein, other electron transport agents and/or layers of
this invention can be obtained by straight-forward application of
the silanation procedure described above in Example 3, with routine
synthetic modification(s) and optimization of reaction conditions
as would be well-known to those skilled in the art and as required
by the particular heterocyclic substrate. Preliminary
halogenation/bromination can be effected using known synthetic
procedures or through choice of starting materials enroute to a
given heterocycle. Alternatively, the heterocycles of FIGS. 4A-4C
and other suitable substrates can be prepared using other available
synthetic procedures to provide multiple silane reaction centers
for use with the self-assembly methods and light-emitting diodes of
this invention. Core molecular substrates of the type from which
the heterocycles of FIGS. 4A-4C can be prepared are also described
by Strukelji et al. in Science, 267, 1969 (1995).
Example 5
With reference to FIGS. 3A-3C and the representative chromophores
provided therein, emissive agents and/or layers, in accordance with
this invention, can be obtained by appropriate choice of starting
materials and using halogenation and silanation procedures of the
type described in Examples 1-4, above. Alternatively, other
chromophores can be silanated using other available synthetic
procedures to provide multiple silane reaction centers for use with
the self-assembly methods and light-emitting diodes of this
invention. Regardless, in accordance with this invention, such
emissive agents or chromophores can be used for emission of light
at wavelengths heretofore unpractical or unavailable. Likewise, the
present invention allows for the use of multiple agents or
chromophores and construction of an emissive layer or layers such
that a combination of wavelengths and/or white light can be
emitted.
Example 6
Examples 6(a)-6(c) together with FIG. 6 illustrate the preparation
of other molecular components which can be used in accordance with
this invention.
Example 6a
Synthesis of Tertiary Arylamine [6]. Together, 14.46 g (20 mmole)
of tris(4-bromophenyl)amine and 500 ml of dry diethyl ether were
stirred at -78.degree. C. under a nitrogen atmosphere. Next, 112.5
ml of a 1.6 M n-butyllithium solution in hexanes was slowly added
to the reaction mixture over 1.5 hours. The reaction was then
warmed to -10.degree. C. and stirred for an additional 30 minutes.
The reaction was then cooled down again to -78.degree. C. before
the addition of 22 g (0.5 mole) of ethylene oxide. The mixture was
stirred and slowly warmed to room temperature over 12 hours. Next,
2 ml of a dilute NH.sub.4Cl solution was then added to the reaction
mixture. The solvent was evaporated under vacuum yielding a light
green solid. The product was purified using column chromatography.
The column was first eluted with chloroform and then with
MeOH:CH.sub.2Cl.sub.2 (5:95 v/v). The resulting light gray solid
was recrystallized using chloroform to give 1.89 g. Yield: 25%.
.sup.1H NMR (.delta., 20.degree. C., DMSO): 2.65 (t, 6H), 3.57 (q,
6H), 4.64 (t, 3H), 6.45 (d, 6H), 7.09 (d, 6H). EI-MS: 377
(M.sup.+), 346 (M.sup.+-31), 315 (M.sup.+-62). HRMS: 377.2002.
calcd; 377.1991. Anal. Calculated for C.sub.24H.sub.27NO.sub.3; C,
76.36; H, 7.21; N, 3.71. Found: C, 76.55; H, 7.01; N, 3.52.
Example 6b
Synthesis of Tosylated Arylamine [7]. A pyridine solution of tosyl
chloride (380 mg in 5 ml) was added over 5 minutes to a pyridine
solution of [6] (500 mg in 10 ml, from Example 6a) cooled to
0.degree. C. The mixture was stirred for 12 hours, then quenched
with water and extracted with chloroform. The organic extract was
washed with water, 5% sodium bicarbonate, and dried with magnesium
sulfate. After filtration, the chloroform solution was then
evaporated to dryness under vacuum and purified using column
chromatography. The column was first eluted with hexane:CHCl.sub.3
(1:2 v/v) yielding [7]. .sup.1H NMR (300 MHz, .delta., 20.degree.
C., CDCl.sub.3): 2.45 (s, 3H), 2.90 (t, 6H), 3.02 (t, 3H), 3.70 (t,
3H), 4.19 (t, 6H), 6.92 (d, 2H), 6.98 (d, 4H), 7.00 (d, 4H), 711
(d, 2H), 7.32 (d, 2H), 7.77 (d, 2H).
Example 6c
Synthesis of Tosylated Arylamine [8]. Continuing the
chromatographic procedure similar for 2 (from Example 6b) but
changing the eluting solvent to 100% CHCl.sub.3 yielded [8].
.sup.1H NMR (300 MHz, .delta., 20.degree. C., CDCl.sub.3): 2.44 (s,
6H), 2.91 (t, 3H), 3.02 (t, 6H), 3.70 (t, 6H), 4.19 (t, 3H), 6.92
(d, 4H), 6.98 (d, 2H), 7.00 (d, 2H), 7.11 (d, 4H), 7.32 (d, 4H),
7.77 (d, 4H).
Example 7
Using the arylamines of Examples 6 and with reference to FIG. 7, an
electroluminescent article/device also in accordance with this
invention is prepared as described, below. It is understood that
the arylamine component can undergo another or a series of
reactions with a silicon/silane moiety of another molecular
component/agent to provide a siloxane bond sequence between
components, agents and/or conductive layers. Similar
electroluminescent articles/devices and conductive layers/media can
be prepared utilizing the various other molecular components/agents
and/or layers described above, such as in Examples 1-5 and FIGS.
1-4, in conjunction with the synthetic modifications of this
invention and as required to provide the components with the
appropriate reactivity and functionality necessary for the assembly
method(s) described herein.
Example 7a
This example of the invention shows how slides can be
prepared/cleaned prior to use as or with electrode materials. An
indium-tin-oxide (ITO)-coated soda lime glass (Delta Technologies)
was boiled in a 20% aqueous solution of ethanolamine for 5 minutes,
rinsed with copious amounts of distilled water and dried for 1 hour
at 120.degree. C.; alternatively and with equal effect, an
ITO-coated soda lime glass (Delta Technologies) was sonicated in
0.5M KOH for 20 minutes, rinsed with copious amounts of distilled
water and then ethanol, and dried for 1 hour at 120.degree. C.
Example 7b
Electroluminescent Article Fabrication and Use. The freshly cleaned
ITO-coated slides were placed in a 1% aqueous solution of
3-aminopropyltrimethoxysilane and then agitated for 5 minutes.
These coated slides were then rinsed with distilled water and cured
for 1 hour at 120.degree. C. The slides were subsequently placed in
a 1% toluene solution of [7] (or [8] from Example 6) and stirred
for 18 hours under ambient conditions. Afterwards, the slides were
washed with toluene and cured for 15 minutes at 120.degree. C.
AlQ.sub.3 (or GaQ.sub.3; Q=quinoxalate) was vapor deposited on top
of the amine-coated slides. Finally, 750-1000 .ANG. of aluminum was
vapor deposited over the metal quinolate layer. Wires were attached
to the Al and ITO layers using silver conducting epoxy
(CircuitWorks.TM.), and when a potential (<7V) was applied, red,
orange, and/or green light was emitted from the device.
Example 8
One or more capping layers comprising
Cl.sub.3SiOSiCl.sub.2OSiCl.sub.3 are successively deposited onto
clean ITO-coated glass where hydrolysis of the deposited material
followed by thermal curing/crosslinking in air at 125.degree. C.
yields a thin (.about.7.8 .ANG.) layer of material on the ITO
surface. X-ray reflectivity measurements indicate that the total
film thickness increases linearly with repeated layer deposition,
as seen in FIG. 8. Other molecular components can be used with
similar effect. Such components include, without limitation, the
bifunctional silicon compounds described in U.S. Pat. No.
5,156,918, at column 7 and elsewhere therein, incorporated by
reference herein in its entirety. Other useful components, in
accordance with this invention include those trifunctional
compounds which cross-link upon curing. As would be well known to
those skilled in the art and made aware of this invention, such
components include those compounds chemically reactive with both
the electrode capped and an adjacent conductive layer.
Example 9
Cyclic voltametry measurements shown in FIG. 9 using aqueous
ferri/ferrocyanide show that there is considerable blocking of the
electrode after the deposition of just one layer of the
self-assembled capping material specified in Example 8. Other
molecular components described, above, show similar utility. Almost
complete blocking, as manifested by the absence of pinholes, is
observed after application of three layers of capping material.
Example 10
Conventional organic electroluminescent devices consisting of TPD
(600 .ANG.)/Alq (600 .ANG.)/Mg (2000 .ANG.) were vapor-deposited on
ITO substrates modified with the capping material specified in
Example 8. FIGS. 10A-C show the behavior of these devices with
varying thickness of the self-assembled capping material. These
results show that such a material can be used to modify forward
light output and device quantum efficiency. For a device with two
capping layers, higher current densities and increased forward
light output are achieved at lower voltages, suggesting an optimum
thickness of capping material can be used to maximize performance
of an electroluminescent article.
Example 11
This example illustrates how a capping material can be introduced
to and/or used in the construction of an electroluminescent
article. ITO-coated glass substrates were cleaned by sonication in
acetone for 1 hour followed by sonication in methanol for 1 hour.
The dried substrates were then reactively ion etched in an oxygen
plasma for 30 seconds. Cleaned substrates were placed in a reaction
vessel and purged with nitrogen. A suitable silane, for instance a
24 mM solution of octachlorotrisiloxane in heptane, was added to
the reaction vessel in a quantity sufficient to totally immerse the
substrates. (Other such compounds include those described in
Example 8). Substrates were allowed to soak in the solution under
nitrogen for 30 minutes. Following removal of the siloxane solution
the substrates were washed and sonicated in freshly distilled
pentane followed by a second pentane wash under nitrogen.
Substrates were then removed from the reaction vessel washed and
sonicated in acetone. Substrates were dried in air at 125.degree.
C. for 15 minutes. This process can be repeated to form a capping
layer of precisely controlled thickness.
Example 12
The stability of device-type TPD-Si.sub.2 molecular layer/TPD hole
transport layer interfaces under thermal stress (one measure of
durability) was investigated by annealing ITO/TPD-Si.sub.2 (40
nm)/TPD (100 nm) bilayers at 80.degree. C. for 1.0 h. The optical
image of the annealed TPD film shows no evidence of TPD
de-wetting/de-cohesion (FIG. 12A), indicating that the ITO-TPD
surface energy mismatch is effectively moderated by the interfacial
TPD-Si.sub.2 molecular layer. In contrast, the bare ITO/TPD
interface exhibits catastrophic de-wetting/de-cohesion under
identical thermal cycling (FIG. 12B), visible even under a layer of
Alq). Despite seemingly similar cohesive effects for both TAA and
TPD-Si.sub.2 as interfacial buffer layers, it is reasonable to
suggest that the interfacial cohesion between TPD-Si.sub.2 and TPD
is greater, given closer structural similarity, evidenced by
comparing advancing aqueous contact angles: values for bare ITO,
silyl-functionalized TAA, silyl (Si.sub.2) functionalized TPD and
TPD film surfaces are 0.degree., 45.degree., 70.degree., and
85.degree. respectively, indicating a closer surface energy match
at TPD-TPD-Si.sub.2 interfaces.
Example 13
Speculation that one role of Cu(Pc) in enhancing OLED performance
might be via the above adhesion mechanism led to parallel thermal
studies. In contrast to a preferred alkylsilyl-substituted
arylamine TPD-Si.sub.2, Cu(Pc)-buffered ITO does not prevent TPD
de-cohesion upon heating to temperatures near/above the TPD glass
transition temperature (T.sub.g). FIG. 12D illustrates the
morphology of a 100 nm TPD film on 10 nm Cu(Pc) following heating
at 80.degree. C. It is clearly seen that thermal annealing induces
TPD crystallization on the Cu(Pc) film surface (visible even under
a layer of Alq), yielding star-shaped dendritic crystallites
(as-deposited TPD films on Cu(Pc) are smooth and featureless, FIG.
12C). It is likely that such Cu(Pc)-nucleated crystallization
occurs during localized heating in operating OLEDs and contributes
to observed device instability.
Example 14
Device characteristics employing spincoated TAA, TPD-Si.sub.2, and
vapor-deposited Cu(Pc) hole injection layer/adhesion layers in
OLEDs having the structure ITO/interlayer/TPD/Alq/Al are compared
in FIG. 13. Versus the bare ITO system, all of the molecular/buffer
layer-incorporated devices exhibit higher light output, enhanced
quantum efficiencies, and lower turn-on voltages. Note that a
preferred TPD-Si.sub.2 component layer affords .about.15,000
cd/m.sup.2 of maximum light output, which is 10-100.times. greater
than the bare ITO-based device. Similar increases in
ITO/TPD/Alq/Al-type device performance with electrode
functionalization have only been reported previously for LiF or
CsF-modified Al cathodes, via what remains an unresolved mechanism.
It is widely accepted that conventional ITO/TPD/Alq/Al
heterostructures are electron-limited due to the low Alq electron
mobility and Alq LUMO-Al Fermi level energetic mismatch, thus
raising the question of why TPD-Si.sub.2 anode modification
produces similar effects. It is suggested that under conditions of
anode-HTL surface energy mismatch and poor anode-HTL cohesion (an
unmodified device of prior art), non-ohmic contacts dominate device
behavior, resulting in significant hole injection barriers typical
of poor electrode-organic contact.
Example 15
With reference to FIG. 14, TAA and TPD-Si.sub.2
interfacial/molecular component layers are significantly more
effective injection structures than conventional Cu(Pc) layers. The
maximum light output for a Cu(Pc)-based device is .about.1500
cd/m.sup.2 at 25 V, while that of a TAA-based device is 2600
cd/m.sup.2, and at a much lower bias voltage (16 V). The external
quantum efficiency of the Cu(Pc)-based device also falls well below
those based on TAA and TPD-Si.sub.2 anode layers: the maximum
quantum efficiency for Cu(Pc) is .about.0.3%, in contrast to those
of TPD-Si.sub.2 (.about.1.2%) and TAA (.about.0.9%). The
TPD-Si.sub.2-functionalized device is most efficient, producing a
maximum light output .about.10.times. greater than the
Cu(Pc)-buffered device, and .about.100.times. greater than the bare
ITO-based device. Improvements observed and differences between
TPD-Si.sub.2 and silyl-functionalized TAA molecular layers can be
explained, without limitation, in relation to: (1) closer aromatic
structural similarity of TPD-Si.sub.2 to TPD, producing a stronger
interlayer affinity and presumably greater .pi.-.pi. interfacial
overlap, and (2) the higher triarylamine:siloxane linker ratio in
TPD-Si.sub.2, consistent with more facile hole hopping via denser
triarylamine packing.
Example 16
These findings argue that promotion of ITO-TPD interfacial
contact/adhesion leads to more efficient hole injection due to
reduced interfacial contact resistance. This hypothesis was tested
by examining characteristics of hole-only devices having the
structure ITO/molecular interlayer/TPD(250 nm)/Au(6 nm)/Al(80 nm),
in which electron injection at the cathode is blocked. Here the Au
layer is deposited by rf-sputtering to avoid excessive heating of
the TPD underlayer. The hole injection capacity falls in the order
TPD-Si.sub.2.gtoreq.TAA>Cu(Pc)>bare ITO (FIG. 14). Compared
to bare ITO, the silyl-functionalized TAA- and
TPD-Si.sub.2-modified anodes enhance the hole current density by
10-100.times. for the same field strength, with ITO/TPD-Si.sub.2
being most effective. Thus, when contact resistance at the OLED
anode side is reduced and hole injection increased, the greater
electric field induced across the Alq layer enhances electron
injection and transport, affording higher light output and
comparable or, in the cases where recombination is more probable,
enhanced quantum efficiency. In contrast, the present and related
data for Cu(Pc) devices show that the Cu(Pc) significantly
suppresses hole injection. E. W. Forsythe, M. A. Abkowitz, Y. Gao,
J. Phys. Chem. B 2000, 104, 3948; S. C. Kim, G. B. Lee, M. Choi, Y.
Roh, C. N. Whang, K. Jeong, Appl. Phys. Lett. 2001, 78, 1445. It is
believed, in light of these results, that Cu(Pc) enhances quantum
efficiency via better balancing hole and electron injection
fluences, rather than by facilitating hole injection or interfacial
stability.
Example 17
To examine cohesion and crystallization effects on device
durability, thermal stress tests were carried out on devices based
on bare ITO, having a 10 nm Cu(Pc) interlayer, and having a 40 nm
TPD-Si.sub.2 interlayer. These were subjected to heating at
95.degree. C. for 0.5 h in vacuum and subsequently examined for
changes in luminous response. The irreversible degradation of the
bare ITO and Cu(Pc)-based devices upon heating at 95.degree. C. for
0.5 h (FIG. 15) is reasonably ascribed to TPD de-wetting and
Cu(Pc)-nucleated TPD crystallization, respectively. Both processes
would disrupt the multilayer structure, leading to direct hole
injection into, and consequent degradation of, the emissive Alq
layer, and possible amplification of pinholes and defects. In
contrast, TPD-Si.sub.2-buffered molecular layer devices exhibit
enhanced performance after heating, which is presumably a
consequence of interfacial reconstruction that promotes charge
injection. These experiments unambiguously demonstrate that
covalently interlinked alkylsilyl-substituted compounds such as
TPD-Si.sub.2 and TAA, when used as described herein, offer
significant improvements in stabilizing the anode-HTL interface and
promoting hole injection.
The results of this and several preceding examples, demonstrate
that a spincoated, hole injecting TPD-Si.sub.2 layer can
significantly increase maximum OLED device luminence
(.about.100.times.) and quantum efficiency (.about.6.times.) by
promoting ITO-TPD interfacial cohesion, hence promoting more
efficient hole injection. Devices having a TPD-Si.sub.2 anode
adhesion layer afford a maximum luminance level of 15,000
cd/m.sup.2 in absence of dopants or low work function cathodes,
while exhibiting excellent thermal stability. In addition, the same
results demonstrate that Cu(Pc) interlayers nucleate TPD
crystallization upon heating above the T.sub.g of TPD
Example 18
The synthesis of alkylsilyl-functionalized TAA is as was previously
described, both herein and in the literature. W. Li, Q. Wang, J.
Cui, H. Chou, T. J. Marks, G. E. Jabbour, S. E. Shaheen, B.
Kippelen, N. Pegyhambarian, P. Dutta, A. J. Richter, J. Anderson,
P. Lee, N. Armstrong, Adv. Mater. 1999, 11, 730. TPD, Alq, and
Cu(Pc) were obtained from Aldrich and purified by gradient
sublimation. All other reagents were used as received unless
otherwise indicated. TPD-Si.sub.2 can be synthesized as provided
elsewhere, herein (see Example 2) or according to FIG. 11B and
Examples 19-21, and further characterized by .sup.1H NMR
spectroscopy and elemental analysis.
Example 19
With reference to FIG. 11B enroute to TPD-Si.sub.2, the synthesis
of 4,4'-bis[(p-bromophenyl)phenylamino)biphenyl (9). To a solution
of tris(dibenzyldeneacetone)dipalladium (0.55 g, 0.6 mmol), and
bis-(diphenylphosphino)ferrocene; (0.50 g, 0.9 mmol) in toluene (50
mL), was added 1,4-dibromobenzene (18.9 g, 0.0800 mol) at
25.degree. C., and the solution stirred under N.sub.2 for 10 min.
Subsequently, sodium tert-butoxide (4.8 g, 0.050 mol) and
N,N'-diphenylbenzidine (6.8 g, 0.020 mol) were added, and the
reaction mixture stirred at 90.degree. C. for 12 h. The reaction
mixture was subsequently cooled to 25.degree. C. and poured into
water. The organic layer was separated, and the aqueous layer was
extracted with toluene (3.times.100 mL). The extract was combined
with the original organic layer, and the solvent was removed in
vacuo to give the crude product. This was purified by
chromatography on silica gel using hexane:ethylene chloride (6:1)
as the eluant. A white solid (6.9 g) was obtained in 50% yield.
.sup.1H NMR (CDCl.sub.3): .delta. 6.99 (d, J=8.8 Hz, 4H), 7.02-7.16
(m, 10H), 7.28 (t, J=7.6 Hz, 4H), 7.34 (d, J=8.8 Hz, 4H), 7.45 (d,
J=8.4 Hz, 4H).
Example 20
With further reference to FIG. 11B enroute to TPD-Si.sub.2, the
synthesis of 4,4'-bis[(p-allylphenyl)'phenylamino]biphenyl (10). To
a stirring, anhydrous ether solution (10 mL) of 1 (1.02 g, 1.58
mmol) under N.sub.2 was added dropwise at 25.degree. C. 1.6 mL (3.5
mmol) n-butyl lithium (2.5 M in hexanes), and the mixture stirred
for 2 h. CuI (0.76 g, 4.0 mmol) was then added, the reaction
mixture cooled to 0.degree. C., and allyl bromide (0.60 g, 5.0
mmol) added in one portion. The solution was stirred for 14 h,
after which time it was quenched with 100 mL saturated aqueous
NH.sub.4.sup.+Cl.sup.- solution, followed by extraction with ether
(3.times.100 mL). The combined ether extracts were washed with
water (2.times.100 mL) and brine (2.times.100 mL), and dried over
anhydrous Na.sub.2SO.sub.4. Following filtration, solvent was
removed in vacuo to yield a yellow oil. Chromatography on silica
gel with hexane:ethylene chloride (4:1) afforded 0.63 g white
solid. Yield, 70%. .sup.1H NMR (CDCl.sub.3) .delta. 3.40 (d, J=10
Hz, 4H), 5.10-5.20 (m, 4H), 6.02 (m, 2H), 6.99-7.10 (m, 2H),
7.10-7.20 (m, 16H), 7.28 (t, J=7.6 Hz, 4H), 7.46 (d, J=8.8 Hz, 4H).
Anal. Calcd for C.sub.42H.sub.36N.sub.2: C, 88.68; H, 6.39; N,
5.23. Found, C, 87.50; H, 6.35; N, 4.93.
Example 21
With reference to FIG. 11B enroute to TPD-Si.sub.2, the synthesis
of 4,4'-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl
(11). To a solution of 2 (0.32 g, 0.55 mol) in 30 mL
CH.sub.2Cl.sub.2 at 25.degree. C. was added a grain of
H.sub.2PtCl.sub.6.xH.sub.2O, followed by HSiCl.sub.3 (0.73 g, 5.5
mmol). The reaction solution was warmed to 30.degree. C. and
stirred for 4 h. Removal of the solvent in vacuum yielded a
dark-yellow oil. A mixture of 50 mL pentane and 10 mL toluene was
then added. The resulting solid was filtered off, and the filtrate
was concentrated under vacuum to a viscous, pale-yellow oil. Yield,
98%. .sup.1H NMR (CDCl.sub.3): .delta.1.45 (t, J=7 Hz, 4H), 1.90
(t, J=7 Hz, 4H), 2.70 (brs, 4H), 6.80-7.80 (m, 26H). Anal. Calcd
for C.sub.42H.sub.38Cl.sub.6N.sub.2Si.sub.2: C, 60.07; H, 4.57.
Found, C, 60.52; H, 4.87.
Example 22
With reference to Examples 19-21, a wide variety of arylalkylamine
molecular components and their silyl-functionalized analogs can be
prepared using straight-forward modifications of the synthetic
techniques described herein. For instance, with reference to
Example 19, diphenylbenzidene can be mono- or dialkylated with the
appropriate haloalkyl reagent to provide the desired arylalkylamine
hole transport layer component. As would also be well known to
those skilled in the art made aware of this invention, the
corresponding silyl-functionalized molecular layer component can be
prepared via mono- or dialkylation with the appropriate dihaloalkyl
reagent followed by subsequent silation, adopting the procedures
illustrated in Examples 20 and 21. Accordingly, by way of further
example, the alkylated mono- and diarylamine components, discussed
above, and their silyl-functionalized analogs can be prepared to
provide the structurally-related molecular and hole transport
layers of this invention, and the enhanced performance and/or hole
injection resulting therefrom.
Example 23
TPD-Si.sub.2 and TAA Thin Film Deposition and Characterization.
Indium tin oxide (ITO) glass sheets with a resistance of
20.OMEGA./.quadrature. from Donnelly Corp. were subjected to a
standard literature cleaning procedure. TAA and TPD-Si.sub.2-based
buffer layers were spincoated onto cleaned ITO surfaces from their
respective toluene solutions (10 mg/mL) at 2 Krpm, followed by
curing in moist air at 110.degree. C. for 15 min. Cyclic
voltammetry of spincoated TPD-Si.sub.2 films on ITO was performed
with a BAS 100 electrochemical workstation (scan rate, 100 mV/s; Ag
wire pseudo-reference electrode, Pt wire counter electrode,
supporting electrolyte, 0.1 M TBAHFP in anhydrous MeCN). For
TPD-Si.sub.2 film contiguity assessment, 1.0 mM ferrocene in 0.1 M
TBAHFP/MeCN was used as the probe. Thermogravimetric analysis (TGA)
was carried out on an SDT 2960 DTA-TGA instrument with a scan rate
of 10.degree. C./min under N.sub.2. TGA sample preparation involved
drop-coating a TPD-Si.sub.2 solution in toluene (10 mM) onto clean
glass substrates under ambient conditions. Following solvent
evaporation, the TPD-Si.sub.2-coated slides were cured at
120.degree. C. for 1 h. Upon cooling, the films were detached from
the glass substrates using a razor blade and collected as powders
for TGA characterization.
Example 24
ITO/Buffer Layer/TPD Interfacial Stability Studies. TPD de-cohesion
analysis of the interfacial structures ITO/buffer layer/TPD (100
nm) (spincoated TPD-Si.sub.2, spincoated TAA, vapor-deposited
Cu(Pc)) were carried out in the following manner. Following vapor
deposition of 50-100 nm TPD films onto the respective buffer
layer-coated ITO substrates, the samples were annealed at
80-100.degree. C. under N.sub.2 for 1.0 h, and the film morphology
subsequently imaged by optical microscopy and AFM.
Example 25
OLED Device Fabrication. OLED devices of the structure:
ITO/interlayer/TPD(50 nm)/Alq(60 nm)/Al(100 nm) were fabricated
using standard vacuum deposition procedures (twin evaporators
interfaced to a <1 ppm O.sub.2 glove box facility). Deposition
rates for organic and metal were 2-4 .ANG./sec and 1-2 .ANG./sec
respectively, at 1.times.10.sup.-6 Torr. The OLED devices were
characterized inside a sealed aluminum sample container under
N.sub.2 using instrumentation described elsewhere. J. Cui, Q.
Huang, Q. Wang, T. J. Marks, Langmuir 2001, 17, 2051; W. Li, Q.
Wang, J. Cui, H. Chou, T. J. Marks, G. E. Jabbour, S. E. Shaheen,
B. Kippelen, N. Pegyhambarian, P. Dutta, A. J. Richter, J.
Anderson, P. Lee, N. Armstrong, Adv. Mater. 1999, 11, 730.
Example 26
Device Thermal Stability Evaluations. OLED devices were subjected
to heating under vacuum at 95.degree. C. for 0.5 h, and were
subsequently evaluated for I-V and L-V characteristics as described
above.
Example 27
Materials and Methods Relating Examples 28-38. All manipulations of
air/moisture-sensitive materials were carried out on a
dual-manifold Schlenk line or in a nitrogen-filled glovebox. Ether
and methylene chloride were distilled before use from
sodium/benzophenone ketyl or calcium hydride, respectively. Toluene
was dried using activated alumina and Q5 columns and tested by
benzyphenone ketyl in ether solution. All reagents were used as
received unless otherwise indicated. NMR spectra were obtained on
Varian VXR-400 or 500 MHz NMR instruments. MS analyses were
conducted on a Micromass Quattro II Triple Quadrupole HPLC/MS/MS
mass spectrometer. Elemental analyses were carried out by Midwest
Microlabs. UV-visible absorption spectra of SAM-coated quartz
plates were obtained on a Cary 1E UV-vis spectrometer. Cyclic
voltammetry was performed with a BAS 100 electrochemical
workstation (SAM-coated ITO with .about.1 cm.sup.2 area working
electrodes, Ag wire pseudo-reference electrode, Pt wire counter
electrode, supporting electrolyte, 0.1 M TBAHFP in anhydrous MeCN).
TBAHFP was recrystallized from an ethylacetate/hexanes solution and
dried in vacuo at 100.degree. C. for 10 h. AFM images were obtained
on a Nanoscope III AFM in the contact mode. Specular x-ray
reflectivity experiments on coated single-crystal Si (111)
substrates were performed on the Naval Research Laboratory X23B
beamline at the National Synchrotron Light Source. Advancing
aqueous angles were measured on SAM-coated ITO substrates
immediately after the self-assembly process.
Example 28
With reference to the compounds of FIG. 11C and the schematic of
FIG. 11D, compounds 12-18 were prepared as follows:
Example 28a
Synthesis of 4,4'-bis[(p-bromophenyl)phenylamino)]biphenyl (12). To
a solution of tris(dibenzyldeneacetone)dipalladium (0.55 g, 0.60
mmol) and bis-(diphenylphosphino)ferrocene (0.50 g, 0.90 mmol) in
50 mL toluene, was added 1,4-dibromobenzene (18.9 g, 0.0800 mol) at
25.degree. C., and the solution stirred under N.sub.2 for 10 min.
Subsequently, sodium tert-butoxide (4.8 g, 0.050 mol) and
N,N'-diphenylbenzidine (6.8 g, 0.020 mol) were added, and the
reaction mixture stirred at 90.degree. C. for 12 h. The reaction
mixture was subsequently cooled to 25.degree. C. and poured into
water. The organic layer was separated, and the aqueous layer was
extracted with toluene (3.times.100 mL). The extract was combined
with the original organic layer, and the solvent was removed under
vacuum to give the resultant crude product. The crude product was
purified by chromatography on silica gel using hexane:ethylene
chloride (6:1) as the eluant. Compound 12 was obtained as a white
solid (6.9 g) in 50% yield. .sup.1H NMR (CDCl.sub.3): .delta. 6.99
(d, J=8.8 Hz, 4H), 7.02-7.16 (m, 10H), 7.28 (t, J=7.6 Hz, 4H), 7.34
(d, J=8.8 Hz, 4H), 7.45 (d, J=8.4 Hz, 4H).
Example 28b
Synthesis of
N,N,N',N'-tetrakis-(p-bromophenyl)-biphenyl-4,4'-diamine (13). To a
stirring chloroform solution (20 mL) of 12 (0.5 g, 0.77 mmol),
tetrabutylammonium tribromide (0.74 g, 1.54 mmol) was added in one
portion at 25.degree. C. The reaction was monitored by TLC (elution
hexane:ether=7:1) and was found to be complete after 0.5 h. The
reaction mixture was then washed with aqueous sodium thiosulfate
and water until pH=7, followed by removing solvent under vacuum to
afford a yellowish solid. Next, 500 mL ether was added and the
solution was washed with water (3.times.100 mL) and brine
(2.times.100 mL), and dried over anhydrous Na.sub.2SO.sub.4.
Following filtration, solvent was removed in vacuum to yield a
white solid Recrystallization from chloroform:hexane (1:10)
afforded 0.49 g of 13 as a white solid. Yield, 79%. .sub.1H NMR
(CDCl.sub.3): .delta. 6.99 (d, J=9 Hz, 8H), 7.11 (d, J=9 Hz 4H),
7.38 (d, J=9 Hz, 8H), 7.47 (d, J=9 Hz, 4H), MS (m/z): 804.8 [M,
100]
Example 28c
Synthesis of
N,N,N',N'-tetrakis-(p-allylphenyl)-biphenyl-4,4'-diamine (14). To a
stirring, anhydrous ether solution (8 mL) of 13 (0.050 g, 0.062
mmol) under inert atmosphere, was added dropwise at 25.degree. C.
n-butyl lithium (1.6 M in hexanes, 0.31 mL, 0.50 mmol), and the
mixture stirred for 2 h. The reaction mixture was then cooled to
0.degree. C. followed by addition of copper iodide(I) (0.1 g, 0.5
mmol). After stirring for 5 min., allyl bromide (0.07 g, 0.6 mmol)
was added in one portion. The solution was warmed up to 25.degree.
C. gradually and stirred for 12 h, after which time it was quenched
with 100 mL saturated aqueous NH.sub.4.sup.+Cl.sup.- solution,
followed by extraction with ether (3.times.100 mL). The combined
ether extracts were washed with water (2.times.100 mL) and brine
(2.times.100 mL), and dried over anhydrous Na.sub.2SO.sub.4.
Following filtration, solvent was removed in vacuum to yield an
oil. Chromatography on silica gel with hexane:methylene chloride
(10:1) afforded 0.027 g of 14 as a white solid. Yield, 68%. .sup.1H
NMR (CDCl.sub.3): .delta. 3.37 (brs, J=10 Hz, 8H), 5.06-5.12 (m,
8H), 5.95-6.03 (m, 4H), 7.08 (brs, 20H), 7.41 (brs, 4H). Anal.
Calcd for C.sub.48H.sub.44N.sub.2: C, 88.83; H, 6.85; N, 4.32.
Found, C, 88.89; H, 6.91; N, 4.28.
Example 28d
Synthesis of
N,N,N',N'-tetrakis[(p-trichlorosilylpropyl)-phenyl]-biphenyl-4,4'-diamine
(15). To a solution of 3 (0.040 g, 0.062 mmol) in 25 mL dry
CH.sub.2Cl.sub.2 at 25.degree. C. under inert atmosphere was added
H.sub.2PtCl.sub.6.xH.sub.2O (0.001 g), followed by trichlorosilane
(0.042 g, 0.31 mmol). The reaction solution was warmed to
30.degree. C. and monitored by NMR until the completion of reaction
after 6 h. Removal of the solvent in vacuum yielded an oil. Next,
20 mL dry toluene was added to the residue and the resulting
solution filtered into a Schlenk flask by cannula. The filtrate was
concentrated under vacuum to give 15 as a pale-yellow oil. Yield,
98%. .sup.1H NMR (benzene-d.sub.6): .delta.0.91 (brs, 4H), 1.56
(brs, 8H), 2.21 (brs, 8H), 6.81-7.39 (m, 24H).
Example 28e
Synthesis of 4-bromo-phenyl-diphenyl-amine (16). To a solution of
tris(dibenzylideneacetone)dipalladium (0.41 g, 0.44 mmol), and
bis-(diphenylphosphino)ferrocene (0.37 g, 0.67 mmol) in 50 mL dry
toluene under inert atmosphere, was added sodium tert-butoxide (4.2
g, 0.04 mol) at 25.degree. C. The mixture was stirred for 15 min
followed by addition of 1,4-dibromobenzene (27.9 g, 0.12 mol), and
stirred for another 15 min. Diphenylamine (5.0 g, 0.029 mmol) were
added, and the reaction mixture was heated to 90.degree. C. for 15
h. The reaction mixture was subsequently cooled to 25.degree. C.
and poured into water. The organic layer was separated, and the
aqueous layer was extracted with toluene (3.times.100 mL). The
extracts were combined with the original organic layer, and the
solvent was removed under vacuum to give the resultant crude
product. The crude product was purified by chromatography on silica
gel using hexane:ethylene chloride (6:1) as the eluant. Compound 16
was obtained as a white solid (5.6 g) in 59% yield. .sup.1H NMR
(CDCl.sub.3): .delta. 6.94 (d, J=8 Hz, 2H), 7.01-7.08 (m, 6H),
7.23-7.27 (m, 4H), 7.32 (d, J=8 Hz, 2H). MS (m/z): 323.2 [M,
100].
Example 28f
Synthesis of 4-allyl-phenyl-diphenyl-amine (17). To a stirring,
anhydrous ether solution (20 mL) of 16 (0.58 g, 1.8 mmol) under
inert atmosphere, n-butyl lithium (1.6 M in hexanes, 1.2 mL, 1.92
mmol) was added slowly at -50.degree. C., and the mixture was
stirred at -50.degree. C. for 15 min, and gradually warmed up to
25.degree. C. After 3 h, copper iodide(I) (0.51 g, 2.7 mmol) was
added followed by dropwise addition of allyl bromide (0.32 g, 2.7
mmol). The solution was stirred for 12 h, followed by quenching
with 100 mL saturated aqueous NH.sub.4.sup.+Cl.sup.- solution and
extraction with ether (3.times.100 mL). The combined ether extracts
were washed with water (2.times.100 mL) and brine (2.times.100 mL),
and dried over anhydrous Na.sub.2SO.sub.4. Following filtration,
solvent was removed in vacuum to yield an oil. Chromatography on
silica gel with hexane:methylene chloride (4:1) afforded 0.18 g of
17 as a colorless-oil. Yield, 35%. .sup.1H NMR (CDCl.sub.3):
.delta. 3.40 (d, J=7.5 Hz, 2H), 5.11-5.32 (m, 2H), 5.98-6.07 (m,
1H), 6.98-7.15 (m, 8H), 7.22-7.37 (m, 6H). Anal. Calcd for
C.sub.21H.sub.19N: C, 88.36; H, 6.72; N, 4.91. Found, C, 88.34; H,
6.10; N, 4.24.
Example 28g
Synthesis of Diphenyl-[4-(3-trichlorosilyl-propyl)-phenyl]-amine
(18). To a solution of 17 (0.18 g, 0.65 mmol) in 25 mL dry
CH.sub.2Cl.sub.2 at 25.degree. C. under inert atmosphere was added
H.sub.2PtCl.sub.6.xH.sub.2O (0.001 g), followed by trichlorosilane
(0.88 g, 6.5 mmol). The reaction solution was warmed to 30.degree.
C. and monitored by NMR until the completion of reaction after 4 h.
Removal of the solvent in vacuum yielded a oil. 20 mL dry toluene
was added to the residue and filtered into a Schlenk flask by
cannula. The filtrate was concentrated under vacuum to give 18 as a
oil. Yield, 98%. .sup.1H NMR (benzene): .delta.0.90 (t, J=8 Hz,
2H), 1.55 (m, 2H), 2.20 (t, J=8 Hz, 2H), 6.75-6.83 (m, 4H),
6.98-7.14 (m, 10H).
Example 29
A series of alkyltrichlorosilyl compounds was synthesized and
purified, as described above (see, more particularly, FIG. 11D).
Self-limited anaerobic chemisorption of these compounds onto smooth
(.about.2.5 nm RMS roughness), plasma-cleaned ITO surfaces was
carried out by immersing ITO substrates in 1.0 mM toluene
solutions, followed by rinsing, drying and curing. Adsorbate
characterization included AFM, aqueous contact angles, optica
spectroscopy, cyclic voltammetry, XPS, UPS, and X-ray reflectivity
(XRR), revealing formation of conformal, largely pinhole-free self
assembled monolayers (SAMs) with sub-nanometer thickness control
and essentially identical aggregate surface energies, ionization
potentials, and coverages (Table 1, below). Protocols for OLED
fabrication and data acquisition are provided in examples
33-38.
The effect of SAM structure on ITO-organic interfacial hole
injection was first investigated by fabricating hole-only devices
(having structures
ITO/SAM/N,N-naphathyl-N,'N-phenyl-biphenyl-4,4'-di-amine (NPB, 400
nm)/Au/Al). Since the only difference in the four types of devices
is SAM molecular structure, the results clearly reveal a
significant structure sensitivity of hole injection across the nano
interfacial region. With reference to the data obtained for the
hole-only devices described in several preceding example(s), hole
current densities at 25 V are .about.0.0004 A/cm.sup.2
(TAA-Si.sub.3)<.about.0.004 A/cm.sup.2
(TAA-Si.sub.1)<.about.0.01 A/cm.sup.2
(TPD-Si.sub.2)<.about.0.04 A/cm.sup.2 (TPD-Si.sub.4); hole
injection fluences vary by 1 to 2 orders of magnitude. The current
densities are somewhat lower than those in OLEDs studied below,
principally due to the thicker HTL (hole transport layer) deposited
in the hole-only devices.
Example 30
OLEDs (having structures
ITO/(SAM)/NPB/tris-(8-hydroxyquinolato)aluminum (AlQ): 1%
diisoamylquinacridone (DlQA)/Al) were next fabricated to examine
SAM structure effects on EL response, which are also significant.
Bare ITO and phenylsilane SAM-coated ITO-based devices were also
fabricated for comparison. In Al cathode OLEDs, luminances at 20
mA/cm.sup.2 (a standard current density for device evaluation) are
200 cd/m.sup.2 (TPD-Si.sub.4)<230 cd/m.sup.2
(TPD-Si.sub.2)<400 cd/m.sup.2 (TAA-Si.sub.1)<570 cd/m.sup.2
(TAA-Si.sub.3). This order of current efficiency is opposite to
that of the hole current densities measured above and can be
understood in terms of electron injection-limited electron-hole
recombination events. Appreciable forward external quantum
efficiency (.eta..sub.ext) variations possibly evidence large,
anode-organic interface effects on OLED charge recombination.
Compared to bare ITO-based devices, SAM-induced OLED performance
enhancement is observed, with the modest phenylsilane SAM
improvement mainly attributable to improved ITO anode-HTL contact
via surface energy matching. Comparison between phenyl and
triarylamine silane SAMs indicates that the latter result in lower
anode-HTL hole injection barriers, agreeing with lower turn-on and
operating voltages at identical luminance.
Example 31
In a second device configuration with enhanced electron injection
and a hole-blocking layer (ITO/(SAM)/NPB/AlQ: 1%
DIQA/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/Li/AgMg),
the hole-electron density imbalance is substantially alleviated,
and more efficient recombination is expected. This is indeed
observed, with the maximum luminance and .eta..sub.ext achieved by
TPD-Si.sub.4-based OLEDs (.about.70 000 cd/M.sup.2 and 2.1%,
respectively) nearly 1 order of magnitude and 5 times greater,
respectively, than the comparable device structure of example 30.
Strong SAM structure-OLED response correlations are again observed,
compared to a phenylsilane based device, with the quantum
efficiency ordering reflecting better recombination balance for the
superior hole injection SAMs. The light output of the
TPD-Si.sub.4-based OLED is .about.1.5 to 3 times brighter than that
of TPD-Si.sub.2 (.about.50 000 cd/m.sup.2), TAA-Si.sub.1 (.about.45
000 cd/m.sup.2), and TAA-Si.sub.3 (.about.23 000 cd/m.sup.2) at
identical bias.
Example 32
Cyclic voltammetry of the SAMs on ITO (TBAHFP/CH.sub.3CN) reveals
E.sub.ox/E.sub.red peak separations increasing in the order 331 mV
(TPD-Si.sub.4)<345 mV (TPD-Si.sub.2)<382 mV
(TAA-Si.sub.1)<466 mV (TAA-Si.sub.3). Such data can be used to
estimate interfacial electron-transfer rates for strongly absorbed
redox-active sites and also reflect absorbate structural
inhomogeneity, interactions, and electrode-redox center spacings.
All other factors being equal, larger peak separations
qualitatively correlate with slower interfacial electron transfer.
Interestingly, this electrochemical index of heterogeneous charge
injection and transport efficiency correlates closely with the
solid-state hole-only device of example 29, with respect to
injection and transport capacity:
TPD-Si.sub.4>TPD-Si.sub.2>TAA-Si.sub.1>TAA-Si.sub.3.
Without limitation to any one theory or mode of operation, the
structural basis for these variations may be associated with
different SAM reorganization energies and different triarylamine
cores having different E.sup.o values. Additionally, the distance
from the triarylamine cores to the ITO surface varies as a function
of molecule geometry and linker density. TAA-Si.sub.3 and
TPD-Si.sub.4 have three or four silyl linkers, respectively, while
TAA-Si.sub.1 and TPD-Si.sub.2 have one or two, respectively. The
former two compounds may predominantly lie flat on the ITO surface,
minimizing the triarylamine-anode distance, while the latter two
components may "stand up," leading to different charge injection
and transport characteristics. The XRR-derived SAM thickness and
roughness data, combined with molecular modeling, show that
TAA-Si.sub.3 in fact anchors largely via one linker rather than
three, similar to situations seen previously, while TPD-Si.sub.4
adopts both "flat" and "upright" orientations, yielding a rough
surface. This can be correlated with the greater charge transport
capacity due to the smaller NAr.sub.3-ITO anode spacing. Finally,
differing intermolecular interactions between triarylamine cores
likely arise from the differing molecular shapes and linker
densities and should also affect interfacial charge injection and
transport.
TABLE-US-00001 TABLE 1 Characteristics of Anode Functionalization
Layers.sup.a TAA-Si.sub.1 TAA-Si.sub.3 TPD-Si.sub.2 TPD-Si.sub.4
.lamda..sub.max (nm) 303 304 352 352 thickness (nm).sup.b 1.2 1.4
1.8 1.6.sup.c RMS roughness (nm).sup.b 0.4 0.7 0.7 1.3 aq contact
angle (deg) 90 87 90 90 E.sub.p,a/E.sub.p,c (V).sup.d 1.180/ 1.200/
1.160/ 1.130/ 0.798 0.734 0.815 0.799 coverage .GAMMA. 4.5 4.2
2.5.sup.f 2.1.sup.f (.times.10.sup.-10 mol/cm.sup.2).sup.d
.DELTA.E.sub.p,1/2 (mV).sup.g 340 460 350 440 JP (eV).sup.h 5.8 6.1
.sup.aExperimental details in Supporting Information. .sup.bFrom
X-ray reflectivity of samples identically deposited on oxide-coated
(111)Si. .sup.cThis parameter is uncertain due to surface
roughness. .sup.dFrom cyclic voltammetry (10 V/s). .sup.eEstimated
by CV (0.1 V/s). .sup.fCV coverage consistent with XRR data
assuming two-electron process. .sup.g0.1 V/s scan rate.
.DELTA.E.sub.p,1/2 > 90.6/n mV, indicating redox site
interactions, site heterogeneity, or both. .sup.hFrom UPS.
Example 33
General self-assembly procedure on ITO, quartz or Si wafer
substrates. Taking TAA-Si.sub.1 as an example, pre-cleaned ITO
substrates were immersed in dry toluene (50 mL) to which 0.5 mL of
compound 18 in dry toluene solution (0.1 M) was added. After
heating at .about.80.degree. C. for 1 h, the toluene solution was
removed by cannula and substrates were rinsed with dry toluene
(2.times.50 mL) and wet acetone. Baking the substrates at
110.degree. C./100 mmHg in a vacuum oven for 1 h completed the
self-assembly process.
Example 34
SAM characterization:cyclic voltammetry. SAM-coated ITO, silver
wire and Pt wire were used as the working electrode, reference
electrode, and counter electrode, respectively. All experiments
were carried out in 0.1 M acetonitrile solution of
tetrabutylammonium hexafluorophosphate as the electrolyte at scan
rate 0.1 V/s, or 10 V/s, respectively.
Example 35
SAM (on Si substrates) characterization: X-ray reflectivity.
TABLE-US-00002 TABLE 2 TPD-Si.sub.2 TPD-Si.sub.4 TAA-Si.sub.3
TAA-Si.sub.1 Electron density (e.ANG.-.sup.3) 0.32-0.35 0.30-0.33
0.32-0.34 0.31 Roughness (.ANG.) 7.4-8.2 12-14 7.5 3.9-4.0
Thickness (.ANG.) 17.7-17.9 16.3 13.6 11.0-11.1 Footprint
(.ANG..sup.2) 51-58 80-98 51-66 44-50 Calculated Coverage .GAMMA.
2.8-3.3 1.7-2.1 2.5-3.0 3.3-3.8 (.times.10.sup.-10 mol/cm.sup.2)*
*Based on electron density profiles obtained from X-ray
reflectivity measurements, the number of electrons per unit of
substrate area for SAMs are calculated as N.sub.SAM =
.intg..rho.(z)dz. The molecular footprints were calculated as
N.sub.mol/N.sub.SAM, where N.sub.mol is the calculated number of
electrons in one molecular unit.
Example 36
SAM characterization: pinhole study by cyclic voltammetry.
SAM-coated ITO, silver wire and Pt wire were used as the working
electrode, reference electrode, and counter electrode,
respectively. All experiments were carried out in 0.1 M
acetonitrile solution of tetrabutylammonium hexafluorophosphate as
the electrolyte and 0.001 M ferrocene as the internal pin hole
probe. Scan rate 0.1 V/s.
Example 37
SAM-coated ITO-based OLED fabrication. The SAM coated substrates
were transferred to a glove box/twin evaporator fabrication
facilities, followed by thermal evaporation at 1.times.10.sup.-7
Torr of NPB (20 nm), AlQ/1% DIQA (50 nm), BCP (20 nm), aluminum
(140 nm), lithium (1 nm), and Mg/Ag (1:9, 100 nm), corresponding to
the desired OLED structures. NPB, AlQ, DIQA, and BCP were purified
by gradient vacuum sublimation before use. The 0.2.times.0.5
cm.sup.2 OLED emitting areas were defined by shadow masks. Light
output and J-V characteristics were measured with a Keithley 2400
source meter and an IL 1700 research radiometer at 25.degree. C.
under ambient atmosphere. External quantum efficiencies and power
efficiencies were estimated from current density vs. voltage and
luminance vs. current density characteristics.
Example 38
SAM-coated ITO-based hole-only device fabrication. NPB (400 nm), Au
(6 nm), and Al (100 nm) were evaporated onto the SAM coated ITO
substrates. They were characterized with the same procedure as
described above.
The preceding examples present evidence for significant OLED
anode-organic interfacial molecular structure effects on hole
injection and/or transfer and EL properties, and show that these
correlate with heterogeneous electron-transfer characteristics.
Chemically tuning the interface structure represents an effective
approach to studying nanoscale injection layers and yields OLEDs
with high brightness (.about.70 000 cd/m.sup.2), low turn-on
voltages (.about.4 V), and high current efficiencies (.about.8
cd/A).
Example 39
With reference to examples 2-3, 28a-g and FIGS. 2G, 11B and 11D,
and the synthetic techniques described elsewhere herein, various
other hole transport compounds of FIGS. 2A, 2C and 2D-F can be
prepared using--in any possible combination--the starting materials
and reagents of Table 3, where notes a-e reference commercial
source or literature preparation. It will be understood in the art
that various other commercially- or synthetically-available
aromatic amines and halides and alkene halides can be used as
described herein or with straight-forward modification of such
techniques, without undue experimentation. Likewise, other
available silane reagents can be employed to provide hydrolyzable
silyl groups and the corresponding silyl-functionalized hole
transport compounds, in accordance with this invention.
TABLE-US-00003 TABLE 3 Reagents for Preparation of
Silyl-Functionalized Aromatic Amine Hole Transport Compounds
Aromatic Amine Aromatic Halide Alkene Halide Silane
2-Naphthalenamine.sup.a 1-Bromo-napthlene.sup.a Allylbromide.sup.a
Trimeth- oxysilane.sup.a Biphenyl amine.sup.a
1,8-Dibromo-anthracene.sup.b 5-Bromo-pentene.sup.a Ch-
lorodimethoxysilane N,N'- 1-Bromo-anthracene.sup.d
Vinylbromide.sup.a Dichloroethoxysilane diphenylbenzidine.sup.a
N-Phenyl-2- 1,8-Dibromo-anthracene.sup.b 4-Bromobutene.sup.a
Trichlorosila- ne.sup.a naphthylamine.sup.a N-2-Naphthyl-1-
Biphenyl bromide.sup.a allylbromide.sup.a Triethoxysilane.sup.a
naphthylamine.sup.e .sup.aAldrich .sup.bHaenel, M. W.; Jakubik, D.;
Krueger, C.; Betz, P. Chem. Ber. 1991, 124, 333-336. .sup.cGelest
.sup.dNetka, J.; Crump, S. L.; Rickborn, B.J. Org. Chem. 1986, 51,
1189-1199. .sup.eASDI Product List
While the principles of this invention have been described in
connection with specific embodiments, it should be understood
clearly that these descriptions are added only by way of example
and are not intended to limit, in any way, the scope of this
invention. For instance, the present invention can be applied more
specifically to the construction of second-order nonlinear optical
materials as have been described in U.S. Pat. No. 5,156,918 which
is incorporated herein by reference in its entirety. Likewise, the
present invention can be used in conjunction with the preparation
of optical waveguides. Another advantages and features will become
apparent from the claims hereinafter, with the scope of the claims
determined by the reasonable equivalents, as understood by those
skilled in the art.
* * * * *